Gas Optimizations

[code423n4]

> 0 is less efficient than != 0 for uint in require condition

Ref: https://twitter.com/GalloDaSballo/status/1485430908165443590

contracts/MarginAccount.sol:150:        require(amount > 0, "Add non-zero margin");

Use custom errors

Solidity ^0.8.4 allow the use of custom errors to optimize gas usage.
https://blog.soliditylang.org/2021/04/21/custom-errors/

Cache array length

contracts/ClearingHouse.sol:122: for (uint i = 0; i < amms.length; i++) {
contracts/ClearingHouse.sol:130: for (uint i = 0; i < amms.length; i++) {
contracts/ClearingHouse.sol:170: for (uint i = 0; i < amms.length; i++) {
contracts/ClearingHouse.sol:194: for (uint i = 0; i < amms.length; i++) { // liquidate all positions
contracts/ClearingHouse.sol:251: for (uint i = 0; i < amms.length; i++) {
contracts/ClearingHouse.sol:263: for (uint i = 0; i < amms.length; i++) {
contracts/ClearingHouse.sol:277: for (uint i = 0; i < amms.length; i++) {

        uint namms = amms.length;
        for (uint i = 0; i < namms; i++) {

Functions can be mark as external

https://github.com/code-423n4/2022-02-hubble/blob/ed1d885d5dbc2eae24e43c3ecbf291a0f5a52765/contracts/ClearingHouse.sol#L148
function liquidateMaker(address maker) override public whenNotPaused {
https://github.com/code-423n4/2022-02-hubble/blob/ed1d885d5dbc2eae24e43c3ecbf291a0f5a52765/contracts/ClearingHouse.sol#L153
function liquidateTaker(address trader) override public whenNotPaused {

Safe math can be put in unchecked block

For example, ClearingHouse.sol#211-215 can be put in unchecked block
https://github.com/code-423n4/2022-02-hubble/blob/ed1d885d5dbc2eae24e43c3ecbf291a0f5a52765/contracts/ClearingHouse.sol#L210

        if (liquidationFee > 0) {
            uint toInsurance = liquidationFee / 2;
            marginAccount.transferOutVusd(address(insuranceFund), toInsurance);
            marginAccount.transferOutVusd(_msgSender(), liquidationFee - toInsurance);
        }

Unnecessary checks

There are implicit assert as commented in the code
https://github.com/code-423n4/2022-02-hubble/blob/ed1d885d5dbc2eae24e43c3ecbf291a0f5a52765/contracts/AMM.sol#L487
https://github.com/code-423n4/2022-02-hubble/blob/ed1d885d5dbc2eae24e43c3ecbf291a0f5a52765/contracts/AMM.sol#L511

Use stuct storage instead of memory

A struct in storage is call by ref which don't need to copy the contents to memory
https://github.com/code-423n4/2022-02-hubble/blob/ed1d885d5dbc2eae24e43c3ecbf291a0f5a52765/contracts/AMM.sol#L528

Unused library

https://github.com/code-423n4/2022-02-hubble/blob/ed1d885d5dbc2eae24e43c3ecbf291a0f5a52765/contracts/Oracle.sol#L13

    using SafeCast for int256;

Variables can be set immutable

https://github.com/code-423n4/2022-02-hubble/blob/ed1d885d5dbc2eae24e43c3ecbf291a0f5a52765/contracts/MarginAccountHelper.sol

    IMarginAccount marginAccount;
    VUSD vusd;
    IERC20 public reserveToken;

[atvanguard]

Good but duplicate.