Gas Optimizations

[code423n4]

Gas Optimizations Report for zkSync v2 contest

Overview

During the audit, 4 gas issues were found.
Total savings ~1500+.

№	Title	Instance Count	Saved
G-1	Use calldata instead of memory for read-only arguments	19	1140
G-2	Use unchecked blocks for incrementing i	4	140
G-3	Use inline function for internal function called once	8	240
G-4	Elements that are smaller than 32 bytes (256 bits) may increase gas usage	12

Gas Optimizations Findings(4)

G-1. Use calldata instead of memory for read-only arguments

Description

Since Solidity v0.6.9, memory and calldata are allowed in all functions regardless of their visibility type (See "Calldata Variables" section here).
When function arguments should not be modified, it is cheaper to use calldata.

Instances

• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L23
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L80
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L190
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L278
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L385
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Mailbox.sol#L43
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Mailbox.sol#L53
• 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/libraries/Diamond.sol#L145
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/libraries/Diamond.sol#L167
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/libraries/Diamond.sol#L277
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/bridge/L1ERC20Bridge.sol#L260
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/bridge/L1EthBridge.sol#L214
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/common/L2ContractHelper.sol#L43
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/common/L2ContractHelper.sol#L47
• https://github.com/code-423n4/2022-10-zksync/blob/main/zksync/contracts/bridge/L2StandardERC20.sol#L43
• https://github.com/code-423n4/2022-10-zksync/blob/main/zksync/contracts/ExternalDecoder.sol#L10
• https://github.com/code-423n4/2022-10-zksync/blob/main/zksync/contracts/ExternalDecoder.sol#L15

Recommendation

Consider using calldata where possible.

Saved

This saves at least 60 gas per iteration.
So, ~60*19 = 1140 #

G-2. Use unchecked blocks for incrementing i

Description

In Solidity 0.8+, there’s a default overflow and underflow check on unsigned integers. In the loops, "i" will not overflow because the loop will run out of gas before that.

Instances

• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/libraries/Diamond.sol#L94
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/libraries/Diamond.sol#L132
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/libraries/Diamond.sol#L153
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/libraries/Diamond.sol#L173

Recommendation

Change:

for (uint256 i; i < n; ++i) {
 // ...
}

to:

for (uint256 i; i < n;) { 
 // ...
 unchecked { ++i; }
}

Saved

This saves ~30-40 gas per iteration.
So, ~35*4 = 140 #

G-3. Use inline function for internal function called once

Description

Function calls need two extra JUMP instructions and other stack operations.

Instances

• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L274
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L349
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L362
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L372
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Executor.sol#L376
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/zksync/facets/Mailbox.sol#L75
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/bridge/L1EthBridge.sol#L113
• https://github.com/code-423n4/2022-10-zksync/blob/main/ethereum/contracts/common/L2ContractHelper.sol#L71

Saved

This saves ~20-40.
So, ~30*8 = 240 #

G-4. Elements that are smaller than 32 bytes (256 bits) may increase gas usage

Description

According to docs, when using elements that are smaller than 32 bytes, your contract’s gas usage may be higher. This is 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.

Instances

• uint8 l2ShardId;
• uint16 txNumberInBlock;
• uint16 txNumberInBlock;
• uint16 selectorPosition;
• uint16 facetPosition;
• uint64 expirationBlock;
• uint192 layer2Tip;
• uint64 blockNumber;
• uint64 indexRepeatedStorageChanges;
• uint64 blockNumber;
• uint64 timestamp;
• uint64 indexRepeatedStorageChanges;

Recommendation

Consider using a larger size where needed.

[GalloDaSballo]

G-1 | Use calldata instead of memory for read-only arguments | 19 | 1140 Most will not compile, ignoring

G-2 | Use unchecked blocks for incrementing i | 4 | 140 200

G-3 | Use inline function for internal function called once | 8 | 240 8*24 192

392

[c4-judge]

GalloDaSballo marked the issue as grade-c