GasTestStorageInMapping.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

/**
 * @title GasTestStorageInMapping
 * @author IllIllI
 */

contract GasTestStorageInMapping {
    struct Foo {
        uint x;
        uint a;
        uint y;
        uint b;
        uint z;
    }
    mapping(uint=>Foo) data;

    constructor() {
        data[100] = Foo({x:0,a:1,y:0,b:2,z:0});
    }

    // 25988 gas (optimizer 200), 26590 gas (optimizer off)
    function bad_6eP(uint idx) external view returns (uint){
        uint a = data[idx].a;
        a *= 2;
        uint b = data[idx].b;
        return a + b;
    }

    // 25946 gas (optimizer 200), 26547 gas (optimizer off)
    function good_H7A(uint idx) external view returns (uint) {
        Foo storage f = data[idx];
        uint a = f.a;
        a *= 2;
        uint b = f.b;
        return a + b;
    }
}