Trader Joe v2 contest - Mathieu's results

Tight Variable Packing

The Tight Variable Packing pattern in Solidity allows the packing of storage variables where possible so that they are stored within the same 32 bytes slot.

In the factory file LBFactory.sol, from line 32 to 39 : https://github.com/code-423n4/2022-10-traderjoe/blob/main/src/LBFactory.sol#L32:L39 we can see 4 storage variables which are not optimally defined in order to benefits from this pattern: LBPairImplementation (address), feeRecipient(address), flashLoanFee(uint256), creationUnlocked(bool).

As per Solidity, here are the following bytes used for the these types:

• address: 20 bytes
• uint256: 32 bytes
• bool: 1 byte

So, with this uint256 variable, we are using 4 slots with each of these variables occupying a slot. The goal here is to ideally pack these 4 variables into 2 slots so we can benefit of lower gas consumption.

In order to achieve this, we would first change the type of the flashLoanFee to a uint88 (11 bytes) because there is no need for this variable to be greater than 309,485,009,821,345,068,724,781,055 as it is expressed in basis point. Then, we just need to apply the change:

address public override LBPairImplementation;

address public override feeRecipient;

uint88 public override flashLoanFee;

/// @notice Whether the createLBPair function is unlocked and can be called by anyone (true) or only by owner (false)
bool public override creationUnlocked;

With this new declaration, LBPairImplementation is stored into 1 slot and feeRecipient, flashLoanFee, creationUnlocked are stored into a second slot of 32 bytes. (20 + 11 + 1)

Of course, the type of the flashLoanFee must be changed to uint88 in every following function and in the event declaration:

• https://github.com/code-423n4/2022-10-traderjoe/blob/main/src/LBFactory.sol#L63:L68 (constructor)
• https://github.com/code-423n4/2022-10-traderjoe/blob/main/src/LBFactory.sol#L474:L481 (setFlashLoanFee)
• https://github.com/code-423n4/2022-10-traderjoe/blob/main/src/interfaces/ILBFactory.sol#L36 (event FlashLoanFeeSet)

In addition to this, the LBPair contract must be modified when doing a flash loan (e.g. flashLoan function), the type of the variable related to the fees must be changed to uint88:

• https://github.com/code-423n4/2022-10-traderjoe/blob/main/src/LBPair.sol#L428
• https://github.com/code-423n4/2022-10-traderjoe/blob/main/src/LBPair.sol#L1044

And the content of the _getFlashLoanFee function requires a slight modification to properly handle the calculation:

function _getFlashLoanFee(uint256 _amount, uint88 _fee) internal pure returns (uint256) {
    return (_amount * uint256(_fee)) / Constants.PRECISION;
}

Other possibility

In case the type of the flashLoanFee variable cannot be reduced to a uint88 for some reason and should remain a uint256, I would just recommend to change the declaration's order of the variables this way:

address public override LBPairImplementation;

address public override feeRecipient;

/// @notice Whether the createLBPair function is unlocked and can be called by anyone (true) or only by owner (false)
bool public override creationUnlocked;

uint256 public override flashLoanFee;

This would simply use 3 storage slots instead of 4.