Only using SafeMath when necessary can save gas

[code423n4]

Handle

WatchPug

Vulnerability details

For the arithmetic operations that will never over/underflow, using SafeMath will cost more gas.

For example:

https://github.com/code-423n4/2021-12-yetifinance/blob/5f5bf61209b722ba568623d8446111b1ea5cb61c/packages/contracts/contracts/BorrowerOperations.sol#L791-L795

if (_debtChange > _variableYUSDFee) { // if debt decrease, and greater than variable fee, decrease 
    newDebt = _troveManager.decreaseTroveDebt(_borrower, _debtChange.sub(_variableYUSDFee));
} else { // otherwise increase by opposite subtraction
    newDebt = _troveManager.increaseTroveDebt(_borrower, _variableYUSDFee.sub(_debtChange));
}

_debtChange - _variableYUSDFee at L792 and _variableYUSDFee - _debtChange at L794 will never underflow.

https://github.com/code-423n4/2021-12-yetifinance/blob/5f5bf61209b722ba568623d8446111b1ea5cb61c/packages/contracts/contracts/YUSDToken.sol#L240-L241

_totalSupply = _totalSupply.add(amount);
_balances[account] = _balances[account].add(amount);

_balances[account] + amount will not overflow if _totalSupply.add(amount) dose not overflow.

https://github.com/code-423n4/2021-12-yetifinance/blob/5f5bf61209b722ba568623d8446111b1ea5cb61c/packages/contracts/contracts/YUSDToken.sol#L248-L249

_balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");
_totalSupply = _totalSupply.sub(amount);

_totalSupply - amount will not underflow if _balances[account].sub(amount) dose not underflow.

[kingyetifinance]

Similar to #85 but not exactly same.