solidity – Does It Cost More Gas to Store Memory Variables in a Struct or Use Scoping

Digging into aaveV2 and uniswapV2 code recently, and I've notice that the two projects solve the stack too deep error problem is different ways.

AaveV2 combines all variables into a struct, and the struct is declared once as a struct of type memory at the beginning of the function.

UniswapV2 on the other hand, uses scoping to force local variables to fall out of scope once they are done being used.

My question is, what are the gas efficiency implications of one solution vs the other? The Aave method seems much more programmer/clean code friendly in my opinion, but it makes me wonder if it is coming at the cost of higher gas consumption.

Here are examples of both methods:

AaveV2:

  struct ValidateBorrowLocalVars {
    uint256 currentLtv;
    uint256 currentLiquidationThreshold;
    uint256 amountOfCollateralNeededETH;
    uint256 userCollateralBalanceETH;
    uint256 userBorrowBalanceETH;
    uint256 availableLiquidity;
    uint256 healthFactor;
    bool isActive;
    bool isFrozen;
    bool borrowingEnabled;
    bool stableRateBorrowingEnabled;
  }

  /**
   * @dev Validates a borrow action
   * @param asset The address of the asset to borrow
   * @param reserve The reserve state from which the user is borrowing
   * @param userAddress The address of the user
   * @param amount The amount to be borrowed
   * @param amountInETH The amount to be borrowed, in ETH
   * @param interestRateMode The interest rate mode at which the user is borrowing
   * @param maxStableLoanPercent The max amount of the liquidity that can be borrowed at stable rate, in percentage
   * @param reservesData The state of all the reserves
   * @param userConfig The state of the user for the specific reserve
   * @param reserves The addresses of all the active reserves
   * @param oracle The price oracle
   */

  function validateBorrow(
    address asset,
    DataTypes.ReserveData storage reserve,
    address userAddress,
    uint256 amount,
    uint256 amountInETH,
    uint256 interestRateMode,
    uint256 maxStableLoanPercent,
    mapping(address => DataTypes.ReserveData) storage reservesData,
    DataTypes.UserConfigurationMap storage userConfig,
    mapping(uint256 => address) storage reserves,
    uint256 reservesCount,
    address oracle
  ) external view {
    ValidateBorrowLocalVars memory vars;

    (vars.isActive, vars.isFrozen, vars.borrowingEnabled, vars.stableRateBorrowingEnabled) = reserve
      .configuration
      .getFlags();

    require(vars.isActive, Errors.VL_NO_ACTIVE_RESERVE);
    require(!vars.isFrozen, Errors.VL_RESERVE_FROZEN);
    require(amount != 0, Errors.VL_INVALID_AMOUNT);

    require(vars.borrowingEnabled, Errors.VL_BORROWING_NOT_ENABLED);

    //validate interest rate mode
    require(
      uint256(DataTypes.InterestRateMode.VARIABLE) == interestRateMode ||
        uint256(DataTypes.InterestRateMode.STABLE) == interestRateMode,
      Errors.VL_INVALID_INTEREST_RATE_MODE_SELECTED
    );

    (
      vars.userCollateralBalanceETH,
      vars.userBorrowBalanceETH,
      vars.currentLtv,
      vars.currentLiquidationThreshold,
      vars.healthFactor
    ) = GenericLogic.calculateUserAccountData(
      userAddress,
      reservesData,
      userConfig,
      reserves,
      reservesCount,
      oracle
    );

    require(vars.userCollateralBalanceETH > 0, Errors.VL_COLLATERAL_BALANCE_IS_0);

    require(
      vars.healthFactor > GenericLogic.HEALTH_FACTOR_LIQUIDATION_THRESHOLD,
      Errors.VL_HEALTH_FACTOR_LOWER_THAN_LIQUIDATION_THRESHOLD
    );

    //add the current already borrowed amount to the amount requested to calculate the total collateral needed.
    vars.amountOfCollateralNeededETH = vars.userBorrowBalanceETH.add(amountInETH).percentDiv(
      vars.currentLtv
    ); //LTV is calculated in percentage

    require(
      vars.amountOfCollateralNeededETH <= vars.userCollateralBalanceETH,
      Errors.VL_COLLATERAL_CANNOT_COVER_NEW_BORROW
    );

    /**
     * Following conditions need to be met if the user is borrowing at a stable rate:
     * 1. Reserve must be enabled for stable rate borrowing
     * 2. Users cannot borrow from the reserve if their collateral is (mostly) the same currency
     *    they are borrowing, to prevent abuses.
     * 3. Users will be able to borrow only a portion of the total available liquidity
     **/

    if (interestRateMode == uint256(DataTypes.InterestRateMode.STABLE)) {
      //check if the borrow mode is stable and if stable rate borrowing is enabled on this reserve

      require(vars.stableRateBorrowingEnabled, Errors.VL_STABLE_BORROWING_NOT_ENABLED);

      require(
        !userConfig.isUsingAsCollateral(reserve.id) ||
          reserve.configuration.getLtv() == 0 ||
          amount > IERC20(reserve.aTokenAddress).balanceOf(userAddress),
        Errors.VL_COLLATERAL_SAME_AS_BORROWING_CURRENCY
      );

      vars.availableLiquidity = IERC20(asset).balanceOf(reserve.aTokenAddress);

      //calculate the max available loan size in stable rate mode as a percentage of the
      //available liquidity
      uint256 maxLoanSizeStable = vars.availableLiquidity.percentMul(maxStableLoanPercent);

      require(amount <= maxLoanSizeStable, Errors.VL_AMOUNT_BIGGER_THAN_MAX_LOAN_SIZE_STABLE);
    }
  }

UniswapV2: (Notice the { and } brackets defining custom scopes)

    // this low-level function should be called from a contract which performs important safety checks
    function swap(uint amount0Out, uint amount1Out, address to, bytes calldata data) external lock {
        require(amount0Out > 0 || amount1Out > 0, 'UniswapV2: INSUFFICIENT_OUTPUT_AMOUNT');
        (uint112 _reserve0, uint112 _reserve1,) = getReserves(); // gas savings
        require(amount0Out < _reserve0 && amount1Out < _reserve1, 'UniswapV2: INSUFFICIENT_LIQUIDITY');

        uint balance0;
        uint balance1;
        { // scope for _token{0,1}, avoids stack too deep errors
        address _token0 = token0;
        address _token1 = token1;
        require(to != _token0 && to != _token1, 'UniswapV2: INVALID_TO');
        if (amount0Out > 0) _safeTransfer(_token0, to, amount0Out); // optimistically transfer tokens
        if (amount1Out > 0) _safeTransfer(_token1, to, amount1Out); // optimistically transfer tokens
        if (data.length > 0) IUniswapV2Callee(to).uniswapV2Call(msg.sender, amount0Out, amount1Out, data);
        balance0 = IERC20(_token0).balanceOf(address(this));
        balance1 = IERC20(_token1).balanceOf(address(this));
        }
        uint amount0In = balance0 > _reserve0 - amount0Out ? balance0 - (_reserve0 - amount0Out) : 0;
        uint amount1In = balance1 > _reserve1 - amount1Out ? balance1 - (_reserve1 - amount1Out) : 0;
        require(amount0In > 0 || amount1In > 0, 'UniswapV2: INSUFFICIENT_INPUT_AMOUNT');
        { // scope for reserve{0,1}Adjusted, avoids stack too deep errors
        uint balance0Adjusted = balance0.mul(1000).sub(amount0In.mul(3));
        uint balance1Adjusted = balance1.mul(1000).sub(amount1In.mul(3));
        require(balance0Adjusted.mul(balance1Adjusted) >= uint(_reserve0).mul(_reserve1).mul(1000**2), 'UniswapV2: K');
        }

        _update(balance0, balance1, _reserve0, _reserve1);
        emit Swap(msg.sender, amount0In, amount1In, amount0Out, amount1Out, to);
    }