Quick gas comparison between multiple ways to access struct elements stored in a single word

test.sol

pragma solidity 0.8.16;
// optim 200 runs
contract Test { //27038
  struct StructA {
    uint128 _a;
    uint128 _b;
  }

  StructA structA;

  constructor() {
    structA._a = uint128(block.timestamp);
    structA._b = uint128(1 hours);
  }


  function test() external returns (uint) {
    return structA._a + structA._b;
  }
}

contract Test2 { //27050
  struct StructA {
    uint128 _a;
    uint128 _b;
  }

  StructA structA;

  constructor() {
    structA._a = uint128(block.timestamp);
    structA._b = uint128(1 hours);
  }


  function test() external returns (uint) {
    (uint128 _a, uint128 _b) = (structA._a, structA._b);
    return _a + _b;
  }
}

contract Test3 { //27123
  struct StructA {
    uint128 _a;
    uint128 _b;
  }

  StructA structA;

  constructor() {
    structA._a = uint128(block.timestamp);
    structA._b = uint128(1 hours);
  }


  function test() external returns (uint) {
    StructA memory _structA = structA;
    return _structA._a + _structA._b;
  }
}

contract Test4 { //27047
  struct StructA {
    uint128 _a;
    uint128 _b;
  }

  StructA structA;

  constructor() {
    structA._a = uint128(block.timestamp);
    structA._b = uint128(1 hours);
  }


  function test() external returns (uint) {
    StructA storage _structA = structA;
    return _structA._a + _structA._b;
  }
}