Caching state variable in local variables for repeated reads saves gas by converting expensive SLOADs into much cheaper MLOADs

[code423n4]

Handle

0xRajeev

Vulnerability details

Impact

SLOADs cost 2100 gas for first time reads of state variables and then 100 gas for repeated reads in the context of a transaction (post Berlin fork). MLOADs cost 3 gas units. Therefore, caching state variable in local variables for repeated reads saves gas.

Proof of Concept

Examples of state variables that are read at the lines shown and also later in that same function:

rewardsPeriod: https://github.com/code-423n4/2021-08-yield/blob/4dc46470e616dd0cbd9db9b4742e36c4d809e02c/contracts/utils/token/ERC20Rewards.sol#L80

_totalSupply: https://github.com/code-423n4/2021-08-yield/blob/4dc46470e616dd0cbd9db9b4742e36c4d809e02c/contracts/utils/token/ERC20Rewards.sol#L107

nextPool: https://github.com/code-423n4/2021-08-yield/blob/4dc46470e616dd0cbd9db9b4742e36c4d809e02c/contracts/yieldspace/Strategy.sol#L163

ladle: https://github.com/code-423n4/2021-08-yield/blob/4dc46470e616dd0cbd9db9b4742e36c4d809e02c/contracts/yieldspace/Strategy.sol#L172

base: https://github.com/code-423n4/2021-08-yield/blob/4dc46470e616dd0cbd9db9b4742e36c4d809e02c/contracts/yieldspace/Strategy.sol#L180

pool (600 gas savings): https://github.com/code-423n4/2021-08-yield/blob/4dc46470e616dd0cbd9db9b4742e36c4d809e02c/contracts/yieldspace/Strategy.sol#L183

pool: https://github.com/code-423n4/2021-08-yield/blob/4dc46470e616dd0cbd9db9b4742e36c4d809e02c/contracts/yieldspace/Strategy.sol#L208

cached: https://github.com/code-423n4/2021-08-yield/blob/4dc46470e616dd0cbd9db9b4742e36c4d809e02c/contracts/yieldspace/Strategy.sol#L262

Tools Used

Manual Analysis

Recommended Mitigation Steps

Consider caching state variables in local variables.

[alcueca]

Fix for ERC20Rewards - _totalSupply Not optimizing for gas in governance actions when it adds lines.

[alcueca]

Fix (Strategy) and another fix