Last Updated on 24. March 2025 by Mario Oettler

In the previous topic, the EVM took care of storing and retrieving the values of two or more variables from a single slot. But it is also possible to do this manually by bit shifting.

Let us have a look at an example.

Example

pragma solidity 0.8.29;

contract PackManually{
    uint64 public varA;


    function setVariables() public{
        // This stores two values 1 and 2 in a single 64 bit variable
        varA = uint64(1) << 32 | uint64(2);
    }

    function readVariables() public returns(uint32, uint32){
        uint32 a = uint32(varA >> 32);
        uint32 b = uint32(varA);
        return (a, b);
    }

}

For comparison, we use the following contract where EVM packing is used.

pragma solidity 0.8.21;

contract PackManualla_Unpacked{
    uint32 public varA;
    uint32 public varB;

    function setVariables() public{
        // This stores two values 1 and 2 in a single 64 bit variable
        varA = 1;
        varB = 2;
    }

    function readVariables() public returns(uint32, uint32){
        return (varA, varB);
    }

}

The gas costs are as follows:

	Deployment	First value assignment	Second value assignment	Read variables
Unpacked	180020	50218	27333	27414
Packed	179827	49878	26993	27402

The downside on this approach is that it compromises readability and increases complexity.