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

[HighBe]

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:

• APIConsumer.sol
  function name: fulfill
  event name:  DataFullfilled
  variable:    volume->_volume

    function fulfill(bytes32 _requestId, uint256 _volume) public recordChainlinkFulfillment(_requestId)
    {
        volume = _volume;
        emit DataFullfilled(volume);
    }

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!

[PatrickAlphaC]

I think I see what you're saying.

Change emit DataFullfilled(volume); -> emit DataFullfilled(_volume);? That makes sense to me

[HighBe]

I think I see what you're saying.

Change emit DataFullfilled(volume); -> emit DataFullfilled(_volume);? That makes sense to me

We really appreciate your response. Yes, changing volume into _volume can save gas according to our observation. We are constantly improving our prototype and also considering whether such (and similar) gas optimizations can be intergeated into the compiler optimization process.