Gas Optimizations

[code423n4]

Unnecessary equals boolean

Boolean variables can be checked within conditionals directly without the use of equality operators to true/false.

Code instances:

    Comptroller.sol, 997: require(msg.sender == admin || state == true, "only admin can unpause");
    Comptroller.sol, 142: if (marketToJoin.accountMembership[borrower] == true) {
    Comptroller.sol, 1011: require(msg.sender == admin || state == true, "only admin can unpause");
    Comptroller.sol, 1029: require(msg.sender == admin || state == true, "only admin can unpause");
    Comptroller.sol, 1065: require(market.isListed == true, "comp market is not listed");

Unused state variables

Unused state variables are gas consuming at deployment (since they are located in storage) and are a bad code practice. Removing those variables will decrease deployment gas cost and improve code quality. This is a full list of all the unused storage variables we found in your code base.

Code instances:

    Comptroller.sol, collateralFactorMaxMantissa
    Comptroller.sol, closeFactorMinMantissa
    Comptroller.sol, closeFactorMaxMantissa

Unused declared local variables

Unused local variables are gas consuming, since the initial value assignment costs gas. And are a bad code practice. Removing those variables will decrease the gas cost and improve code quality. This is a full list of all the unused storage variables we found in your code base.

Code instances:

    Comptroller.sol, distributeBorrowerComp, borrowState
    Comptroller.sol, updateCompSupplyIndex, compAccrued
    Comptroller.sol, liquidateCalculateSeizeTokens, exchangeRateMantissa
    Comptroller.sol, distributeSupplierComp, supplyState
    Comptroller.sol, updateCompBorrowIndex, borrowAmount

Change transferFrom to transfer

'transferFrom(address(this), *, *)' could be replaced by the following more gas efficient 'transfer(, **)' This replacement is more gas efficient and improves the code quality.

Code instances:

    CNft.sol, 152 : IERC721(underlying).safeTransferFrom(address(this), msg.sender, tokenIds[i], "");
    CNft.sol, 141 : IERC1155(underlying).safeBatchTransferFrom(address(this), msg.sender, tokenIds, amounts, "");

Unnecessary array boundaries check when loading an array element twice

There are places in the code (especially in for-each loops) that loads the same array element more than once. 
In such cases, only one array boundaries check should take place, and the rest are unnecessary.
Therefore, this array element should be cached in a local variable and then be loaded
again using this local variable, skipping the redundant second array boundaries check: 

Code instance:

    Comptroller.sol.claimComp - double load of holders[j]

Caching array length can save gas

Caching the array length is more gas efficient. This is because access to a local variable in solidity is more efficient than query storage / calldata / memory. We recommend to change from:

for (uint256 i=0; i<array.length; i++) { ... }

to:

uint len = array.length  
for (uint256 i=0; i<len; i++) { ... }

Code instances:

    CEther.sol, , 178
    ERC1155Enumerable.sol, ids, 51
    CNft.sol, vars, 176
    Comptroller.sol, assets, 591
    Comptroller.sol, allMarkets, 928

Prefix increments are cheaper than postfix increments

Prefix increments are cheaper than postfix increments. Further more, using unchecked {++x} is even more gas efficient, and the gas saving accumulates every iteration and can make a real change There is no risk of overflow caused by increamenting the iteration index in for loops (the ++i in for (uint256 i = 0; i < numIterations; ++i)). But increments perform overflow checks that are not necessary in this case. These functions use not using prefix increments (++x) or not using the unchecked keyword:

Code instances:

    just change to unchecked: CNft.sol, i, 176
    just change to unchecked: CNft.sol, i, 98
    change to prefix increment and unchecked: Comptroller.sol, i, 591
    change to prefix increment and unchecked: Comptroller.sol, i, 1223
    change to prefix increment and unchecked: Comptroller.sol, i, 199

Unnecessary index init

In for loops you initialize the index to start from 0, but it already initialized to 0 in default and this assignment cost gas. It is more clear and gas efficient to declare without assigning 0 and will have the same meaning:

Code instances:

    Comptroller.sol, 928
    Comptroller.sol, 1223
    Comptroller.sol, 949
    Comptroller.sol, 591
    Comptroller.sol, 1240

Unnecessary default assignment

Unnecessary default assignments, you can just declare and it will save gas and have the same meaning.

Code instances:

    Comptroller.sol (L#81) : uint internal constant collateralFactorMaxMantissa = 0.9e18; 
    Comptroller.sol (L#75) : uint internal constant closeFactorMinMantissa = 0.05e18; 
    Comptroller.sol (L#78) : uint internal constant closeFactorMaxMantissa = 0.9e18; 

Short the following require messages

The following require messages are of length more than 32 and we think are short enough to short them into exactly 32 characters such that it will be placed in one slot of memory and the require function will cost less gas. The list:

Code instances:

    Solidity file: CNft.sol, In line 93, Require message length to shorten: 35, The message: CNFT: Liquidator cannot be borrower
    Solidity file: Comptroller.sol, In line 996, Require message length to shorten: 39, The message: only pause guardian and admin can pause
    Solidity file: Comptroller.sol, In line 702, Require message length to shorten: 34, The message: cNFT is from the wrong comptroller
    Solidity file: Comptroller.sol, In line 171, Require message length to shorten: 37, The message: exitMarket: getAccountSnapshot failed
    Solidity file: Comptroller.sol, In line 995, Require message length to shorten: 40, The message: cannot pause a market that is not listed

Unnecessary Reentrancy Guards

Where there is onlyOwner or Initializer modifer, the reentrancy gaurd isn't necessary (unless you don't trust the owner or the deployer, which will lead to full security breakdown of the project and we believe this is not the case) This is a list we found of such occurrences:

Code instance:

    CNft.sol no need both nonReentrant and onlyOwner modifiers in call

Use != 0 instead of > 0

Using != 0 is slightly cheaper than > 0. (see https://github.com/code-423n4/2021-12-maple-findings/issues/75 for similar issue)

Code instances:

    CErc20.sol, 194: change 'balance > 0' to 'balance != 0'
    CNft.sol, 127: change 'balance > 0' to 'balance != 0'
    Comptroller.sol, 302: change 'redeemAmount > 0' to 'redeemAmount != 0'
    Comptroller.sol, 1255: change 'amount > 0' to 'amount != 0'

Use unchecked to save gas for certain additive calculations that cannot overflow

You can use unchecked in the following calculations since there is no risk to overflow:

Code instance:

    UniswapV2PriceOracle.sol (L#32) - pairObservations[pair][numPairObservations[pair]++ % OBSERVATION_BUFFER_SIZE] = Observation( block.timestamp, px0Cumulative, px1Cumulative );

Consider inline the following functions to save gas

You can inline the following functions instead of writing a specific function to save gas.
(see https://github.com/code-423n4/2021-11-nested-findings/issues/167 for a similar issue.)

Code instances

    Comptroller.sol, adminOrInitializing, { return msg.sender == admin || msg.sender == comptrollerImplementation; }
    Comptroller.sol, getAccountLiquidityInternal, { return getHypotheticalAccountLiquidityInternal(account, address(0), 0, 0); }

Inline one time use functions

The following functions are used exactly once. Therefore you can inline them and save gas and improve code clearness.

Code instances:

    Comptroller.sol, _addMarketInternal
    UniswapV2PriceOracle.sol, price0Cumulative
    UniswapV2PriceOracle.sol, price0
    Comptroller.sol, getAccountLiquidityInternal
    Comptroller.sol, setCompSpeedInternal

Cache powers of 10 used several times

You calculate the power of 10 every time you use it instead of caching it once as a constant variable and using it instead. Fix the following code lines:

Code instance:

CNftPriceOracle.sol, 98 : You should cache the used power of 10 as constant state variable since it's used several times (3): return FullMath.mulDiv(pricePerToken, 1018 - mintFee, 1018);

Change if -> revert pattern to require

Change if -> revert pattern to 'require' to save gas and improve code quality, if (some_condition) { revert(revert_message) }

to: require(!some_condition, revert_message)

In the following locations:

Code instance:

    Comptroller.sol, 303

Upgrade pragma to at least 0.8.4

Using newer compiler versions and the optimizer gives gas optimizations and additional safety checks are available for free.

The advantages of versions 0.8.* over <0.8.0 are:

    1. Safemath by default from 0.8.0 (can be more gas efficient than library based safemath.)
    2. Low level inliner : from 0.8.2, leads to cheaper runtime gas. Especially relevant when the contract has small functions. For example, OpenZeppelin libraries typically have a lot of small helper functions and if they are not inlined, they cost an additional 20 to 40 gas because of 2 extra jump instructions and additional stack operations needed for function calls.
    3. Optimizer improvements in packed structs: Before 0.8.3, storing packed structs, in some cases used an additional storage read operation. After EIP-2929, if the slot was already cold, this means unnecessary stack operations and extra deploy time costs. However, if the slot was already warm, this means additional cost of 100 gas alongside the same unnecessary stack operations and extra deploy time costs.
    4. Custom errors from 0.8.4, leads to cheaper deploy time cost and run time cost. Note: the run time cost is only relevant when the revert condition is met. In short, replace revert strings by custom errors.

Code instances:

    CErc20.sol
    CEther.sol
    Comptroller.sol

Do not cache msg.sender

We recommend not to cache msg.sender since calling it is 2 gas while reading a variable is more.

Code instances:

    https://github.com/code-423n4/bunker-protocol/tree/main/contracts/Comptroller.sol#L85
    https://github.com/code-423n4/bunker-protocol/tree/main/contracts/CEther.sol#L29