Gas Optimizations - Githubissues

Use Storage Instead of Memory for Structs/Arrays

A storage pointer is cheaper since copying a state struct in memory would incur as many SLOADs and MSTOREs as there are slots. In another words, this causes all fields of the struct/array to be read from storage, incurring a Gcoldsload (2100 gas) for each field of the struct/array, and then further incurring an additional MLOAD rather than a cheap stack read. As such, declaring the variable with the storage keyword and caching any fields that need to be re-read in stack variables will be much cheaper, involving only Gcoldsload for all associated field reads. Read the whole struct/array into a memory variable only when it is being returned by the function, passed into a function that requires memory, or if the array/struct is being read from another memory array/struct. Here are some of the instances entailed:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/DiamondProxy.sol#L26 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/libraries/Diamond.sol#L90 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/DiamondInit.sol#L43

calldata and memory

When running a function we could pass the function parameters as calldata or memory for variables such as strings, bytes, structs, arrays etc. If we are not modifying the passed parameter, we should pass it as calldata because calldata is more gas efficient than memory.

Calldata is a non-modifiable, non-persistent area where function arguments are stored, and behaves mostly like memory. However, it alleviates the compiler from the abi.decode() step that copies each index of the calldata to the memory index, bypassing the cost of 60 gas on each iteration. Here are some of the instances entailed:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/libraries/Diamond.sol#L89 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/libraries/Diamond.sol#L121 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L80

Unchecked SafeMath Saves Gas

"Checked" math, which is default in ^0.8.0 is not free. The compiler will add some overflow checks, somehow similar to those implemented by SafeMath. While it is reasonable to expect these checks to be less expensive than the current SafeMath, one should keep in mind that these checks will increase the cost of "basic math operation" that were not previously covered. This particularly concerns variable increments in for loops. When no arithmetic overflow/underflow is going to happen, unchecked { ++i ;} to use the previous wrapping behavior further saves gas just as in the for loop below as an example:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/libraries/Diamond.sol#L173-L179

        for (uint256 i = 0; i < selectorsLength;) {
            bytes4 selector = _selectors[i];
            SelectorToFacet memory oldFacet = ds.selectorToFacet[selector];
            require(oldFacet.facetAddress != address(0), "a2"); // Can't delete a non-existent facet

            _removeOneFunction(oldFacet.facetAddress, selector);

            unchecked {
                ++i;
            }
        }

Non-strict inequalities are cheaper than strict ones

In the EVM, there is no opcode for non-strict inequalities (>=, <=) and two operations are performed (> + = or < + =). As an example, consider replacing >= with the strict counterpart > in the following line of code:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/DiamondCut.sol#L49-L50

        bool approvedBySecurityCouncil = s.diamondCutStorage.securityCouncilEmergencyApprovals >
            SECURITY_COUNCIL_APPROVALS_FOR_EMERGENCY_UPGRADE - 1;

Similarly, as an example, consider replacing <= with the strict counterpart < in the following line of code:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L268

        require(currentTotalBlocksVerified < s.totalBlocksCommitted + 1, "q");

`abi.encode()` Costs More Gas Than `abi.encodePacked()`

Changing abi.encode() to abi.encodePacked() can save gas considering the former pads extra null bytes at the end of the call data, which is unnecessary. Please visit the following the link delineating how abi.encodePacked() is more gas efficient in general:

https://github.com/ConnorBlockchain/Solidity-Encode-Gas-Comparison

Here are some of the instances entailed:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/DiamondCut.sol#L27 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/DiamondCut.sol#L61 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L85 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L182

Payable Access Control Functions Costs Less Gas

Consider marking functions with access control as payable. This will save 20 gas on each call by their respective permissible callers for not needing to have the compiler check for msg.value. Here are some of the instances entailed:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/DiamondCut.sol#L46 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/DiamondCut.sol#L78 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L225

Function Order Affects Gas Consumption

The order of function will also have an impact on gas consumption. Because in smart contracts, there is a difference in the order of the functions. Each position will have an extra 22 gas. The order is dependent on method ID. So, if you rename the frequently accessed function to more early method ID, you can save gas cost. Please visit the following site for further information:

https://medium.com/joyso/solidity-how-does-function-name-affect-gas-consumption-in-smart-contract-47d270d8ac92

Activate the Optimizer

Before deploying your contract, activate the optimizer when compiling using “solc --optimize --bin sourceFile.sol”. By default, the optimizer will optimize the contract assuming it is called 200 times across its lifetime. If you want the initial contract deployment to be cheaper and the later function executions to be more expensive, set it to “ --optimize-runs=1”. Conversely, if you expect many transactions and do not care for higher deployment cost and output size, set “--optimize-runs” to a high number.

module.exports = {
  solidity: {
    version: "0.8.0",
    settings: {
      optimizer: {
        enabled: true,
        runs: 1000,
      },
    },
  },
};

Please visit the following site for further information:

https://docs.soliditylang.org/en/v0.5.4/using-the-compiler.html#using-the-commandline-compiler

Here's one example of instance on opcode comparison that delineates the gas saving mechanism:

for !=0 before optimization PUSH1 0x00 DUP2 EQ ISZERO PUSH1 [cont offset] JUMPI

after optimization DUP1 PUSH1 [revert offset] JUMPI

Disclaimer: There have been several bugs with security implications related to optimizations. For this reason, Solidity compiler optimizations are disabled by default, and it is unclear how many contracts in the wild actually use them. Therefore, it is unclear how well they are being tested and exercised. High-severity security issues due to optimization bugs have occurred in the past . A high-severity bug in the emscripten -generated solc-js compiler used by Truffle and Remix persisted until late 2018. The fix for this bug was not reported in the Solidity CHANGELOG. Another high-severity optimization bug resulting in incorrect bit shift results was patched in Solidity 0.5.6. Please measure the gas savings from optimizations, and carefully weigh them against the possibility of an optimization-related bug. Also, monitor the development and adoption of Solidity compiler optimizations to assess their maturity.

+= and -= Costs More Gas

+= generally costs 22 more gas than writing out the assigned equation explicitly. The amount of gas wasted can be quite sizable when repeatedly operated in a loop. As an example, the following line of code could be rewritten as:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/DiamondCut.sol#L29

        s.diamondCutStorage.currentProposalId =  s.diamondCutStorage.currentProposalId + 1;

The following line of code should logically be refactored as:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/libraries/Merkle.sol#L34

            _index = _index / 2;

`uint256` Can be Cheaper Than `uint8` and Other Unsigned Integer Type of Smaller Bit Size

When dealing with function arguments or memory values, there is no inherent benefit because the compiler does not pack these values. Your contract’s gas usage may be higher because the EVM operates on 32 bytes at a time. Therefore, if the element is smaller than that, the EVM must use more operations in order to reduce the size of the element from 32 bytes to the desired size. The EVM needs to properly enforce the limits of this smaller type.

It is only more efficient when you can pack variables of uint8, uint16, uint32, uint64, ... into the same storage slot with other neighboring variables smaller than 32 bytes. Here are some of the instances entailed:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L367 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/DiamondInit.sol#L30

Use of Named Returns for Local Variables Saves Gas

You can have further advantages in term of gas cost by simply using named return values as temporary local variable. There are numerous instances entailed throughout all codebases:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L83 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L296 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L349

As an example, the following code block can be refactored as:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L279

    function _getBlockProofPublicInput(
        bytes32 _prevBlockCommitment,
        bytes32 _currentBlockCommitment,
        ProofInput calldata _proof,
        VerifierParams memory _verifierParams
    ) internal pure returns (uint256 zkProof) {
        zkProof =
            uint256(
                keccak256(
                    abi.encodePacked(
                        _prevBlockCommitment,
                        _currentBlockCommitment,
                        _verifierParams.recursionNodeLevelVkHash,
                        _verifierParams.recursionLeafLevelVkHash,
                        _verifierParams.recursionCircuitsSetVksHash,
                        _proof.recurisiveAggregationInput
                    )
                )
            ) & INPUT_MASK;
    }

Ternary Over `if ... else`

Using ternary operator instead of the if else statement saves gas. For instance the following code block may be rewritten as:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/libraries/Diamond.sol#L278-L289

    _init == address(0)
        ? {
            require(_calldata.length == 0, "H"); // Non-empty calldata for zero address
        }
        : {
            // Do not check whether `_init` is a contract since later we check that it returns data.
            (bool success, bytes memory data) = _init.delegatecall(_calldata);
            require(success, "I"); // delegatecall failed

            // Check that called contract returns magic value to make sure that contract logic
            // supposed to be used as diamond cut initializer.
            require(data.length == 32, "lp");
            require(abi.decode(data, (bytes32)) == DIAMOND_INIT_SUCCESS_RETURN_VALUE, "lp1");
        }

Internal Function Only Called Once May Be Embedded to Save Gas

In L1ERC20Bridge.sol, _depositFunds() may be embedded in deposit() both reduce contract size and incur less gas on related function calls:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/bridge/L1ERC20Bridge.sol#L116 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/bridge/L1ERC20Bridge.sol#L136-L146

Similarly, _getERC20Getters() may be embedded inline with _getDepositL2Calldata():

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/bridge/L1ERC20Bridge.sol#L155 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/bridge/L1ERC20Bridge.sol#L164

Split Require Statements Using &&

Instead of using the && operator in a single require statement to check multiple conditions, using multiple require statements with 1 condition per require statement will save 3 GAS per &&. Here are some of the instances entailed:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/common/AllowList.sol#L97-L101 https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/common/L2ContractHelper.sol#L65

Private/Internal Function Embedded Modifier Reduces Contract Size

Consider having the logic of a modifier embedded through a private (doesn't matter whether or not the contract entails any child contracts since the private visibility saves even more gas on function calls than the internal visibility) function to reduce contract size if need be. For instance, the following instance of modifier may be rewritten as follows:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/common/ReentrancyGuard.sol#L38-L41

    function _reentrancyGuardInitializer() private view {
        _initializeReentrancyGuard();
    }

    modifier reentrancyGuardInitializer() {
        _reentrancyGuardInitializer();
        _;
    }

`||` Costs Less Gas Than Its Equivalent `&&`

Rule of thumb: (x && y) is (!(!x || !y))

Even with the 10k Optimizer enabled: ||, OR conditions cost less than their equivalent &&, AND conditions.

As an example, the following code line may be rewritten as:

https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/common/L2ContractHelper.sol#L65

        require(!(version != 1 || _bytecodeHash[1] != bytes1(0)), "zf");

code-423n4 / 2022-10-zksync-findings

Gas Optimizations #25

Use Storage Instead of Memory for Structs/Arrays

calldata and memory

Unchecked SafeMath Saves Gas

Non-strict inequalities are cheaper than strict ones

`abi.encode()` Costs More Gas Than `abi.encodePacked()`

Payable Access Control Functions Costs Less Gas

Function Order Affects Gas Consumption

Activate the Optimizer

+= and -= Costs More Gas

`uint256` Can be Cheaper Than `uint8` and Other Unsigned Integer Type of Smaller Bit Size

Use of Named Returns for Local Variables Saves Gas

Ternary Over `if ... else`

Internal Function Only Called Once May Be Embedded to Save Gas

Split Require Statements Using &&

Private/Internal Function Embedded Modifier Reduces Contract Size

`||` Costs Less Gas Than Its Equivalent `&&`

Use Storage Instead of Memory for Structs/Arrays

calldata and memory

code-423n4 / 2022-10-zksync-findings

Gas Optimizations #25

Use Storage Instead of Memory for Structs/Arrays

calldata and memory

Unchecked SafeMath Saves Gas

Non-strict inequalities are cheaper than strict ones

abi.encode() Costs More Gas Than abi.encodePacked()

Payable Access Control Functions Costs Less Gas

Function Order Affects Gas Consumption

Activate the Optimizer

+= and -= Costs More Gas

uint256 Can be Cheaper Than uint8 and Other Unsigned Integer Type of Smaller Bit Size

Use of Named Returns for Local Variables Saves Gas

Ternary Over if ... else

Internal Function Only Called Once May Be Embedded to Save Gas

Split Require Statements Using &&

Private/Internal Function Embedded Modifier Reduces Contract Size

|| Costs Less Gas Than Its Equivalent &&

Use Storage Instead of Memory for Structs/Arrays

calldata and memory

`abi.encode()` Costs More Gas Than `abi.encodePacked()`

`uint256` Can be Cheaper Than `uint8` and Other Unsigned Integer Type of Smaller Bit Size

Ternary Over `if ... else`

`||` Costs Less Gas Than Its Equivalent `&&`