🧱Gas Saving Technique 14: struct layout/ variable packing

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Introduction

Gas optimization in smart contracts is essential for creating efficient and cost-effective applications on the Ethereum blockchain. One subtle yet impactful optimization technique is tight packing of variables within structs to minimize storage slots usage. Proper arrangement and ordering of variables in your structs can save storage slots, reducing the contract's deployment and state-changing transaction costs.

Impact & Details

Understanding Storage Slots

• Storage Slot Consumption: In Ethereum, storage is organized into 32-byte slots. Each slot can hold a single variable of up to 32 bytes, and larger variables can span multiple slots. The ordering of variables in structs can affect how these slots are filled, influencing the gas cost for deploying and interacting with contracts.

• Tight Variable Packing: By strategically ordering variables in structs, developers can ensure that storage slots are utilized efficiently, minimizing the total slots needed.

Implementation of Tight Variable Packing for Gas Savings

Practical Example: Optimizing Struct Variable Layout

Here is an example struct before and after optimization:

Before Optimization:

solidityCopy codestruct Claim {
    uint256 created;
    uint256 updated;
    address initiator;
    bytes32 protocol;
    uint256 amount;
    address receiver;
    uint32 timestamp;
    State state;
    bytes ancillaryData;
}

After Optimization:

solidityCopy codestruct Claim {
    uint256 created;
    uint256 updated;
    uint256 amount;
    address initiator;
    bytes32 protocol;
    address receiver;
    uint32 timestamp;
    State state;
    bytes ancillaryData;
}

In the optimized version, uint256 variables are grouped together at the beginning of the struct, followed by address and smaller-sized variables. This rearrangement allows for more efficient packing of data into storage slots, saving gas during deployment and operation.

Recommended Mitigation Steps

1. Identify Structs for Optimization: Review your smart contracts to identify structs that can benefit from variable reordering.

2. Rearrange Variables for Tight Packing: Reorder variables within each struct to minimize storage slot usage. Group larger variables together and place smaller variables adjacent to each other to make efficient use of the available space within each slot.

3. Test: After making changes, rigorously test the smart contract to verify that its functionality remains intact while it utilizes fewer storage slots and consumes less gas.

Conclusion

Tight variable packing in structs is a simple but effective optimization for gas savings in smart contract development. By minimizing storage slot usage through efficient variable ordering, developers can reduce the gas costs associated with deploying and running smart contracts on the Ethereum network. As always, thorough testing is crucial to ensure that these optimizations do not inadvertently affect the contract's intended functionality.