Gas: Conditional flow optimization in Exchange.sol:removeLiquidity()

[code423n4]

Handle

Dravee

Vulnerability details

Impact

It's possible to save gas by optimizing conditional flows to avoid some unnecessary opcodes

Proof of Concept

In Exchange.sol:removeLiquidity(), the code is as follows:

File: Exchange.sol
225:         if (quoteTokenQtyToReturn > internalBalances.quoteTokenReserveQty) {
226:             internalBalances.quoteTokenReserveQty = 0;
227:         } else {
228:             internalBalances.quoteTokenReserveQty -= quoteTokenQtyToReturn;
229:         }

However, this can be optimized :

• Strict inequalities (>) are more expensive than non-strict ones (>=). This is due to some supplementary checks (ISZERO, 3 gas)
• In this case here, if quoteTokenQtyToReturn == internalBalances.quoteTokenReserveQty: internalBalances.quoteTokenReserveQty = 0 should be used
• Avoiding the else clause would avoid some opcodes (1 SUB = 3 gas, 2 MLOADs = 6 gas...)

The code would become:

File: Exchange.sol
225:         if (quoteTokenQtyToReturn >= internalBalances.quoteTokenReserveQty) {
226:             internalBalances.quoteTokenReserveQty = 0;
227:         } else {
228:             internalBalances.quoteTokenReserveQty -= quoteTokenQtyToReturn;
229:         }

Tools Used

VS Code

Recommended Mitigation Steps

Use the non-strict greater-than operator in this particular case

[GalloDaSballo]

After some back and forth it really seems like >= is more gas efficient (3 gas)
Finding is valid