Use Local Memory Type Variable Instead of Global Storage Type Variable in Event to Save Gas

[Ether1oop]

Hi, we recently have conducted a systematic study about Solidity event usage, evolution, and impact, and we are attempting to build a tool to improve the practice of Solidity event use based on our findings. We have tried our prototype tool on some of the most popular GitHub Solidity repositories, and for your repository, we find a potential optimization of gas consumption arisen from event use.

The point is that when we use emit operation to store the value of a certain variable, local memory type variable would be preferable to global storage type (state) variable if they hold the same value. The reason is that an extra SLOAD operation would be needed to access the variable if it is storage type, and the SLOAD operation costs 800 gas.

For your repository, we find that the following event use can be improved:

• vDODOToken.sol
  function name: updateDODOFeeBurnRatio
  event name:  UpdateDODOFeeBurnRatio
  variable:    DODO_FEE_BURN_RATIO->dodoFeeBurnRatio

    function updateDODOFeeBurnRatio(uint256 dodoFeeBurnRatio) public onlyOwner {
        _DODO_FEE_BURN_RATIO_ = dodoFeeBurnRatio;
        emit UpdateDODOFeeBurnRatio(_DODO_FEE_BURN_RATIO_);
    }

• DODOIncentive.sol
  function name: changePerReward
  event name:  SetPerReward
  variable:    dodoPerBlock->_dodoPerBlock

    function changePerReward(uint256 _dodoPerBlock) public onlyOwner {
        _updateTotalReward();
        dodoPerBlock = _dodoPerBlock;
        emit SetPerReward(dodoPerBlock);
    }

  function name: changeDefaultRate
  event name:  SetDefaultRate
  variable:    defaultRate->_defaultRate

    function changeDefaultRate(uint256 _defaultRate) public onlyOwner {
        defaultRate = _defaultRate;
        emit SetDefaultRate(defaultRate);
    }

  function name: changeDODOProxy
  event name:  SetNewProxy
  variable:    DODO_PROXY->_dodoProxy

    function changeDODOProxy(address _dodoProxy) public onlyOwner {
        _DODO_PROXY_ = _dodoProxy;
        emit SetNewProxy(_DODO_PROXY_);
    }

• RewardVault.sol
  function name: syncValue
  event name:  DepositReward
  variable:    REWARD_RESERVE->rewardBalance

    function syncValue() external {
        uint256 rewardBalance = IERC20(_REWARD_TOKEN_).balanceOf(address(this));
        uint256 rewardInput = rewardBalance.sub(_REWARD_RESERVE_);

        _TOTAL_REWARD_ = _TOTAL_REWARD_.add(rewardInput);
        _REWARD_RESERVE_ = rewardBalance;

        emit DepositReward(_TOTAL_REWARD_, rewardInput, _REWARD_RESERVE_);
    }

Do you find our results useful? Your reply and invaluable suggestions would be greatly appreciated, and are vital for improving our tool. Thanks a lot for your time!