Cache the return value from rewardPerToken()

[code423n4]

Handle

GiveMeTestEther

Vulnerability details

Impact

Save gas by caching the return value from rewardPerToken() in a local variable and use the local variable L220 and L222. This saves us two storage reads (1 cold = 800 gas, and 1 warm= 100 gas). It is way cheaper to read from a local variable (push/pop operations 2-3 gas each + cheap others)

Note: same for the claimReward() function on L555

Proof of Concept

https://github.com/code-423n4/2021-11-streaming/blob/56d81204a00fc949d29ddd277169690318b36821/Streaming/src/Locke.sol#L203

https://github.com/code-423n4/2021-11-streaming/blob/56d81204a00fc949d29ddd277169690318b36821/Streaming/src/Locke.sol#L555

Tools Used

Recommended Mitigation Steps

• cache in a local variable: uint256 _rewardPerToken = rewardPerToken();
• write the value to the storage variable: cumulativeRewardPerToken = _rewardPerToken;
• replace the occurrences of cumulativeRewardPerToken on L220/222 with _rewardPerToken