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:

• KrakenPriceTicker.sol
  function name: __callback
  event name:  LogNewKrakenPriceTicker
  variable:    priceETHXBT->_result

    function __callback(
        bytes32 _myid,
        string memory _result,
        bytes memory _proof
    )
        public
    {
        require(msg.sender == provable_cbAddress());
        update(); // Recursively update the price stored in the contract...
        priceETHXBT = _result;
        emit LogNewKrakenPriceTicker(priceETHXBT);
    }

• WolframAlpha.sol
  function name: __callback
  event name:  LogNewTemperatureMeasure
  variable:    temperature->_result

    function __callback(
        bytes32 _myid,
        string memory _result
    )
        public
    {
        require(msg.sender == provable_cbAddress());
        temperature = _result;
        emit LogNewTemperatureMeasure(temperature);
        // Do something with the temperature measure...
    }

• YoutubeViews.sol
  function name: __callback
  event name:  LogYoutubeViewCount
  variable:    viewsCount->_result

    function __callback(
        bytes32 _myid,
        string memory _result
    )
        public
    {
        require(msg.sender == provable_cbAddress());
        viewsCount = _result;
        emit LogYoutubeViewCount(viewsCount);
        // Do something with viewsCount, like tipping the author if viewsCount > X?
    }

• WolframAlpha.sol
  function name: __callback
  event name:  LogNewTemperatureMeasure
  variable:    temperature->_result

• CallerPaysForQuery.sol
  function name: __callback
  event name:  LogNewEthPrice
  variable:    ethPriceInUSD->_result

• KrakenPriceTicker.sol
  function name: __callback
  event name:  LogNewKrakenPriceTicker
  variable:    priceETHXBT->_result

• YoutubeViews.sol
  function name: __callback
  event name:  LogYoutubeViewCount
  variable:    viewsCount->_result

• EncryptedQuery.sol
  function name: __callback
  event name:  LogNewRequestStatus
  variable:    requestStatus->_result

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!

[adityakode]

wow ! loved your analysis!