Uint Types Gas Comparison

It's a common belief that in Solidity, smaller integer types like uint8, uint16, uint32, uint64, uint128, and uint256 might save gas due to their smaller size. However, this isn't always the case.

The Ethereum Virtual Machine (EVM) allocates a 256-bit slot for each stored variable. For instance, if we declare a variable of type uint8, the EVM fills the missing bits with zeros to fit it into a single slot. Additionally, during execution, the EVM converts uintN types to uint256 for computations.

DemoCode

Let's test uint8, uint32, and uint256 to observe their behavior in terms of writing data.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

contract Uint8Example {
    uint8 public uint8val;

    // gas 22234
    function set() external {
        uint8val = 1;
    }

    // gas 53427
    function compute() external returns (uint8) {
        uint8 target = 50;
        for (uint8 i = 0; i < target; i++) {
            uint8val += 1;
        }
        return uint8val;
    }
}

contract Uint32Example {

    uint32 public uint32val;

    // gas 22234
    function set() external {
        uint32val = 1;
    }

    // gas 53895
    function compute() external returns (uint32) {
        uint32 target = 50;
        for (uint32 i = 0; i < target; i++) {
            uint32val += 1;
        }
        return uint32val;
    }
}

contract Uint256Example {
    uint256 public uint256val;

    // gas 22238
    function set() external {
        uint256val = 1;
    }

    // gas 42950
    function compute() external returns (uint256) {
        uint256 target = 50;
        for (uint256 i = 0; i < target; i++) {
            uint256val += 1;
        }
        return uint256val;
    }
}

As observed, in loop computations, uint256 saves over 10,000 gas. Therefore, smaller variables don't necessarily equate to gas savings.

Recommendations for gas optimization:

🌟If variables cannot be packed together, using uint or uint256 is the optimal choice.