Struct Ordering Rule

Just like Pack Rule, by fitting variables into the fewest possible storage slots—can result in significant gas savings.

Example

pragma solidity ^0.8.17;

contract PackStruct {
    struct PoorlyOrderedStruct {
        address router;
        uint256 amount;
        uint64 timestamp;
    }

    struct WellOrderedStruct {
        uint64 timestamp;
        address router;
        uint256 amount;
    }

    PoorlyOrderedStruct poorExample = PoorlyOrderedStruct(address(0), 100, uint64(block.timestamp));
    WellOrderedStruct goodExample = WellOrderedStruct(uint64(block.timestamp), address(0), 100);

    function readGoodExample() external view returns (WellOrderedStruct memory) {
        return goodExample;
    }

    function readPoorExample() external view returns (PoorlyOrderedStruct memory) {
        return poorExample;
    }
}

Calling function storereadBadStruct would cost 1462 gas and calling function readGoodStruct would cost 4309 gas only, proving that the struct ordering can significantly impact gas consumption.

Test Environment

• Compiler: solc 0.8.17
• Testing Framework: Foundry

ETH saved over 100 transactions

100 tx * 30 gwei * 0.000000001 * (4309-1462) = 0.008541 ~= 0.009 ETH¹

USD Saved over 100 transactions

• At \$4000 / ETH: \$36
• At \$3000/ETH: \$27
• At \$2000/ETH: \$19