Gas Optimizations

[code423n4]

[G-01] Comparison operators
In the EVM, there is no opcode for >= or <=. When using greater than or equal, two operations are performed: > and =.
Using strict comparison operators hence saves gas.

Instances include:
https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/lender/Lender.sol#L712

Mitigation:
Replace <= with <, and >= with >. Do not forget to increment/decrement the compared variable.
However, when 1 is negligible compared to the variable.

[G-02] Use nested multiple if instead of && operator
If statements including conditions with the && operator can be broken down in multiple if statements to save gas.

Instances include:
https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/lender/Lender.sol#L200
https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/redeemer/Redeemer.sol#L168
https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/redeemer/Redeemer.sol#L169
https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/redeemer/Redeemer.sol#L170

[G-03] custom errors
Custom errors from Solidity 0.8.4 are cheaper than revert strings (cheaper deployment cost and runtime cost when the revert condition is met) while providing the same amount of information, as explained here
https://blog.soliditylang.org/2021/04/21/custom-errors/
Custom errors are defined using the error statement

Instances include:
https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/lender/Lender.sol#L710
https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/lender/Lender.sol#L712

Mitigation:
Replace require statements with custom errors.

[G-04] Use 'address(0)' instead of '0x00000000000000000000000000000000000000000000000000000000000000'
https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/lender/Lender.sol#L353

[G-05] ++i costs less gas than i++, especially when it's used in for loops
https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/lender/Lender.sol#L96
https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/lender/Lender.sol#L120
https://github.com/code-423n4/2022-06-illuminate/blob/912be2a90ded4a557f121fe565d12ec48d0c4684/lender/Lender.sol#L289