Gas Optimizations

[code423n4]

C4-001 : Upgrade pragma to at least 0.8.4

Impact - Gas Optimization

Using newer compiler versions and the optimizer gives gas optimizations
and additional safety checks are available for free.

The advantages of versions 0.8.* over <0.8.0 are:

• Safemath by default from 0.8.0 (can be more gas efficient than
  library based safemath.)
• Low level inliner : from 0.8.2, leads to cheaper runtime gas. Especially relevant when the contract has small functions. For example, OpenZeppelin libraries typically have a lot of small helper functions and if they are not inlined, they cost an additional 20 to 40 gas because of 2 extra jump instructions and additional stack operations needed for function calls.
• Optimizer improvements in packed structs: Before 0.8.3, storing packed structs, in some cases used an
  additional storage read operation. After EIP-2929, if the slot was already cold, this means unnecessary stack operations and extra deploy time costs. However, if the slot was already warm, this means
  additional cost of 100 gas alongside the same unnecessary stack operations and extra deploy time costs.
• Custom errors from 0.8.4, leads to cheaper deploy time cost and run time cost. Note: the run time cost is only relevant when the revert condition is met. In short, replace revert strings by custom errors.

Proof of Concept

1. The contest repository contracts contain floating pragma 0.8.0. The contracts pragma version can be updated to 0.8.4 for the gas optimization.

• https://github.com/code-423n4/2022-02-anchor/blob/main/contracts/cross-chain-contracts/ethereum/CrossAnchorBridge.sol#L2

All Contracts

Tools Used

None

Recommended Mitigation Steps

Consider to upgrade pragma to at least 0.8.4.

C4-002 : ++i is more gas efficient than i++ in loops forwarding

Impact

++i is more gas efficient than i++ in loops forwarding.

Proof of Concept

1. Navigate to the following contracts.

https://github.com/code-423n4/2022-02-anchor/blob/main/contracts/cross-chain-contracts/ethereum/CrossAnchorBridge.sol#L149

Tools Used

Code Review

Recommended Mitigation Steps

It is recommend to use unchecked{++i} and change i declaration to uint256.

C4-003 : Cache array length in for loops can save gas

Impact

Reading array length at each iteration of the loop takes 6 gas (3 for mload and 3 to place memory_offset) in the stack.

Caching the array length in the stack saves around 3 gas per iteration.

Proof of Concept

1. Navigate to the following smart contract line.

https://github.com/code-423n4/2022-02-anchor/blob/main/contracts/cross-chain-contracts/ethereum/CrossAnchorBridge.sol#L149

Tools Used

None

Recommended Mitigation Steps

Consider to cache array length.

C4-004 : Redundant Code

Impact

The several code sections are redundant. The variable/structs should be deleted for the gas optimization if It is not used.

Proof of Concept

1. Navigate to the following smart contract line.

https://github.com/code-423n4/2022-02-anchor/blob/main/contracts/cross-chain-contracts/ethereum/CrossAnchorBridge.sol#L19

https://github.com/code-423n4/2022-02-anchor/blob/main/contracts/cross-chain-contracts/ethereum/CrossAnchorBridge.sol#L26

2. Struct Signature and VM is not used.

Tools Used

None

Recommended Mitigation Steps

Consider removing the unused function/structs which is not used.

[GalloDaSballo]

Seems like it would save less than 100 gas
Also doesn't cover any rust contract