The Graph L2 bridge contest - aysha's results

Upgrade Pragma to at Least 0.8.4 Using newer compiler versions and the optimizer give gas optimizations. Also, additional safety checks are available for free.

Use Custom Errors Instead of Revert Strings to Save Gas Custom errors from Solidity 0.8.4 are cheaper than revert strings (cheaper deployment cost and runtime cost when the revert condition is met)

Source: https://blog.soliditylang.org/2021/04/21/custom-errors/:

Starting from Solidity v0.8.4, there is a convenient and gas-efficient way to explain to users why an operation failed through the use of custom errors. Until now, you could already use strings to give more information about failures (e.g., revert("Insufficient funds.");), but they are rather expensive, especially when it comes to deploy cost, and it is difficult to use dynamic information in them.

Custom errors are defined using the error statement, which can be used inside and outside of contracts (including interfaces and libraries).

I suggest replacing revert strings with custom errors.

Splitting require() statements that use && saves gas See this issue: code-423n4/2022-01-xdefi-findings#128 which describes the fact that there is a larger deployment gas cost, but with enough runtime calls, the change ends up being cheaper https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/governance/Governed.sol#L54 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/staking/Staking.sol#L369

It costs more gas to initialize non-constant/non-immutable variables to zero than to let the default of zero be applied Not overwriting the default for stack variables saves 8 gas. Storage and memory variables have larger savings https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/staking/Staking.sol#L924 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/staking/Staking.sol#L979 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/staking/Staking.sol#L1050 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/staking/Staking.sol#L1454 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/staking/Staking.sol#L1475

= costs less gas than > The compiler uses opcodes GT and ISZERO for solidity code that uses >, but only requires LT for >=, which saves 3 gas https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/l2/gateway/L2GraphTokenGateway.sol#L146 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/l2/gateway/L2GraphTokenGateway.sol#L238 ==========================================================

Using > 0 costs more gas than != 0 when used on a uint in a require() statement This change saves 6 gas https://aws1.discourse-cdn.com/business6/uploads/zeppelin/original/2X/3/363a367d6d68851f27d2679d10706cd16d788b96.png per instance. The optimization works until solidity version 0.8.13 https://gist.github.com/IllIllI000/bf2c3120f24a69e489f12b3213c06c94 where there is a regression in gas costs. https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/l2/gateway/L2GraphTokenGateway.sol#L146 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/l2/gateway/L2GraphTokenGateway.sol#L238

Using bools for storage incurs overhead Booleans are more expensive than uint256 or any type that takes up a full word because each write operation emits an extra SLOAD to first read the slot's contents, replace the bits taken up by the boolean, and then write back. This is the compiler's defense against contract upgrades and pointer aliasing, and it cannot be disabled. https://github.com/OpenZeppelin/openzeppelin-contracts/blob/58f635312aa21f947cae5f8578638a85aa2519f5/contracts/security/ReentrancyGuard.sol#L23-L27 Use uint256(1) and uint256(2) for true/false to avoid a Gwarmaccess (100 gas) for the extra SLOAD, and to avoid Gsset (20000 gas) when changing from 'false' to 'true', after having been 'true' in the past https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/governance/Pausable.sol#L8-L10 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/l2/token/GraphTokenUpgradeable.sol#L51

require()/revert() strings longer than 32 bytes cost extra gas Each extra memory word of bytes past the original 32 incurs an MSTORE: https://gist.github.com/hrkrshnn/ee8fabd532058307229d65dcd5836ddc#consider-having-short-revert-strings which costs 3 gas https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/gateway/GraphTokenGateway.sol#L21 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/governance/Managed.sol#L53 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/upgrades/GraphProxy.sol#L105 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/upgrades/GraphProxy.sol#L141-L144 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/upgrades/GraphUpgradeable.sol#L32

Prefix Increments Prefix increments are cheaper than postfix increments - 6 gas. This can mean interesting savings in for loops. Change variable++ to ++variable. https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/staking/Staking.sol#L924 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/staking/Staking.sol#L1050

Calldata Instead Of Memory For Function Parameters If a reference type function parameter is read-only, it is cheaper in gas to use calldata instead of memory. Calldata is a non-modifiable, non-persistent area where function arguments are stored, and behaves mostly like memory,but it alleviates the compiler from the abi.decode() step that copies each index of the calldata to the memory index, each iteration costing 60 gas. https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/governance/Managed.sol#L173 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/l2/gateway/L2GraphTokenGateway.sol#L266 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/l2/gateway/L2GraphTokenGateway.sol#L286 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/staking/Staking.sol#L647

Events Indexing Events should use the maximum amount of indexed fields: up to three parameters. This makes it easier to filter for specific values in front-ends. https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/governance/Managed.sol#L33-L39 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/governance/Pausable.sol#L19-L21 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/l2/gateway/L2GraphTokenGateway.sol#L58-L62 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/l2/token/L2GraphToken.sol#L24-L30 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/staking/Staking.sol#L37-L100 https://github.com/code-423n4/2022-10-thegraph/blob/309a188f7215fa42c745b136357702400f91b4ff/contracts/staking/Staking.sol#L172-L188