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Submission Details
Severity: gas
Valid

Use smaller data types for `lenderFee` and `borrowerFee` for better storage packing

thekmj

Summary

lenderFee and borrowerFee can be packed into smaller data types to save gas.

Vulnerability Details

The current storage layout for Lender.sol is as follow:

uint256 public lenderFee = 1000; // slot 1
uint256 public borrowerFee = 50; // slot 2
address public feeReceiver; // slot 3

All of these slots are initialized to non-zero during construction, incurring 21000 gas each for using a Gsset operation (for 63000 gas in total).

Note that lenderFee and borrowerFee cannot exceed and , respectively. Therefore we can use smaller data types to pack all of those values into one slot.

// all these takes a single slot
uint48 public lenderFee = 1000;
uint48 public borrowerFee = 50;
address public feeReceiver;

This converts two Gsset (21000 gas) into two Gsreset (2900 gas), for a total of 34200 gas per lendings created.

Impact

Saves 34200 gas per lendings created.

Tools Used

Manual review

Recommendations

Use the recommended storage packing.

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