Gas Optimizations

[code423n4]

Title: Using SafeMath for solidity >0.8

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/helpers/ConnextPriceOracle.sol#L45
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/AmplificationUtils.sol#L15

Recommended Mitigation Steps:
it's better to remove using SafeMath for uint256 for solidity >0.8
reference: OpenZeppelin/openzeppelin-contracts#2465

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Title: Caching length for loop can save gas

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/helpers/ConnextPriceOracle.sol#L150

Recommended Mitigation Steps:
Change to:

    uint256 Length = tokenAddresses.length;
    for (uint256 i = 0; i < Length; i++) {

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Title: Using != is more gas efficient

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/helpers/ConnextPriceOracle.sol#L150
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/SwapUtils.sol#L369

Recommended Mitigation Steps:
Change to !=

    require(baseTokenPrice != 0, "invalid base token");

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Title: Using unchecked and prefix increment is more effective for gas saving:

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/helpers/ConnextPriceOracle.sol#L176
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/SwapUtils.sol#L1055

Recommended Mitigation Steps:

    for (uint256 i = 0; i < tokenAddresses.length;) {
      aggregators[tokenAddresses[i]] = AggregatorV3Interface(sources[i]);
      emit AggregatorUpdated(tokenAddresses[i], sources[i]);
    	unchecked{
        ++i; //@audit-info: Place here with unchecked
        }
      }
  }

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Title: Using multiple require instead && can save gas

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/SwapUtils.sol#L397
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/SwapUtils.sol#L1007
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/helpers/StableSwap.sol#L84-L87

Recommended Mitigation Steps:
Change to:

	require(tokenIndexFrom < numTokens, "Tokens must be in pool");
	require(tokenIndexTo < numTokens, "Tokens must be in pool");

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Title: Reduce the size of error messages (Long revert Strings)

Impact:
Shortening revert strings to fit in 32 bytes will decrease deployment time gas and will decrease runtime gas when the revert condition is met.
Revert strings that are longer than 32 bytes require at least one additional mstore, along with additional overhead for computing memory offset, etc.

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/LibDiamond.sol#L121
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/LibDiamond.sol#L123
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/LibDiamond.sol#L139
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/LibDiamond.sol#L141
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/LibDiamond.sol#L236

Recommended Mitigation Steps:
Consider shortening the revert strings to fit in 32 bytes

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Title: Custom errors from Solidity 0.8.4 are cheaper than revert strings

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/LibDiamond.sol
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/SwapUtils.sol

Recommended Mitigation Steps:
I suggest replacing revert strings with custom errors.
reference: https://blog.soliditylang.org/2021/04/21/custom-errors/

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Title: Comparison operators

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/SwapUtils.sol#L925
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/libraries/AmplificationUtils.sol#L84-L85

Recommended Mitigation Steps:
Replace <= with <, and >= with > for gas opt

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Title: using delete statement can save gas

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/promise/PromiseRouter.sol#L251

Recommended Mitigation Steps:
Change to:

    delete callbackFees[transferId];

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Title: Gas improvement on returning sponsoredFee value

Proof of Concept:
https://github.com/code-423n4/2022-05-backd/blob/main/protocol/contracts/Controller.sol#L121-L130

Recommended Mitigation Steps:
by set sponsoredFee in returns and delete L#201 can save gas

 function reimburseLiquidityFees(
    address _token,
    uint256 _liquidityFee,
    address _receiver
  ) external override onlyConnext returns (uint256 sponsoredFee) { //@audit-info: set here
     //@audit-info: remove this line

    if (address(tokenExchanges[_token]) != address(0)) {
      uint256 currentBalance = address(this).balance;
      ITokenExchange tokenExchange = tokenExchanges[_token];

      uint256 amountIn = tokenExchange.getInGivenExpectedOut(_token, _liquidityFee);
      amountIn = currentBalance >= amountIn ? amountIn : currentBalance;

      // sponsored fee may end being less than _liquidityFee due to slippage
      sponsoredFee = tokenExchange.swapExactIn{value: amountIn}(_token, msg.sender);
    } else {
      uint256 balance = IERC20(_token).balanceOf(address(this));
      sponsoredFee = balance < _liquidityFee ? balance : _liquidityFee;

      // some ERC20 do not allow to transfer 0 amount
      if (sponsoredFee > 0) {
        IERC20(_token).safeTransfer(msg.sender, sponsoredFee);
      }
    }

    emit ReimburseLiquidityFees(_token, sponsoredFee, _receiver);

    return sponsoredFee;
  }

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Title: Use of uint8 in for loop increases gas costs

Proof of Concept:
https://github.com/code-423n4/2022-03-lifinance/blob/main/src/Facets/HopFacet.sol#L48

Recommended Mitigation Steps:
Change from uint8 to uint256

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Title: Prefix increments are cheaper than postfix increments.

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/facets/DiamondLoupeFacet.sol#L31

Recommended Mitigation Steps:
Change i++ to ++i

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Title: Use routerBalance that already been cache instead

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/facets/PortalFacet.sol#L108

Recommended Mitigation Steps:
Change to:

	routerBalance -= amountIn;

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Title: Using unchecked can save gas

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/facets/PortalFacet.sol#L147

Recommended Mitigation Steps:
Because of the condition in L#146

unchecked{
  uint256 missing = total - amount;
}

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Title: Consider make constant as private to save gas

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/facets/BridgeFacet.sol#L68

Recommended Mitigation Steps:
I suggest changing the visibility from public to internal or private

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Title: Declare total inside for loop can save gas

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/facets/RelayerFacet.sol#L163

Recommended Mitigation Steps:
can save 3 gas

for (uint256 i; i < _transferIds.length; ) {
      uint256 totall; //@audit-info: set here
      total += s.relayerFees[_transferIds[i]];
      s.relayerFees[_transferIds[i]] = 0;
      unchecked {
        i++;
      }
    }

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Title: Default value initialization

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/facets/VersionFacet.sol#L16
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/facets/BridgeFacet.sol#L68

Recommended Mitigation Steps:
Remove explicit initialization for default values.

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Title: >= is cheaper than >

Impact:
Strict inequalities (>) are more expensive than non-strict ones (>=). This is due to some supplementary checks (ISZERO, 3 gas)

Proof of Concept:
https://github.com/code-423n4/2022-06-connext/blob/main/contracts/contracts/core/connext/helpers/SponsorVault.sol#L256-L258

Recommended Mitigation Steps:
Consider using >= instead of > to avoid some opcodes

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