Gas optimization for L2 in Blockchain / Solidity - Mini Project: Build & Apply

Gas Optimization for Layer 2 Blockchain Transactions

📖 Scenario: You are developing a smart contract that will run on a Layer 2 (L2) blockchain solution. L2 solutions help reduce gas fees and increase transaction speed compared to Layer 1 (L1) blockchains. However, writing efficient smart contract code is still important to minimize gas costs.In this project, you will create a simple contract that stores user balances and allows deposits and withdrawals. You will then optimize the contract code to reduce gas usage by applying common gas-saving techniques.

🎯 Goal: Build a smart contract that manages user balances with deposit and withdrawal functions. Then optimize the contract to reduce gas consumption by using efficient data types, minimizing storage writes, and using unchecked math where safe.

📋 What You'll Learn

Create a mapping to store user balances

Add a deposit function to increase user balance

Add a withdraw function to decrease user balance

Optimize the contract to reduce gas usage

💡 Why This Matters

🌍 Real World

Layer 2 blockchains are used to make blockchain transactions faster and cheaper. Writing gas-efficient smart contracts helps users save money and improves network performance.

💼 Career

Blockchain developers must optimize smart contracts for gas efficiency to build scalable decentralized applications and reduce costs for users.

1

Create the initial balance mapping

Create a Solidity contract named GasOptimizedL2 and inside it, declare a public mapping called balances that maps address to uint256.

Blockchain / Solidity

contract GasOptimizedL2 {
    // Your code here
}

Hint

Use mapping(address => uint256) public balances; inside the contract.

2

Add deposit and withdraw functions

Add two public functions inside GasOptimizedL2: deposit and withdraw. The deposit function should accept a uint256 amount and add it to balances[msg.sender]. The withdraw function should accept a uint256 amount and subtract it from balances[msg.sender] only if the balance is sufficient.

Blockchain / Solidity

contract GasOptimizedL2 {
    mapping(address => uint256) public balances;

    // Your code here
}

Hint

Use balances[msg.sender] += amount; in deposit and check balance with require in withdraw.

3

Optimize gas usage in withdraw function

Modify the withdraw function to use an unchecked block when subtracting amount from balances[msg.sender] to save gas, since the require already ensures no underflow.

Blockchain / Solidity

contract GasOptimizedL2 {
    mapping(address => uint256) public balances;

    function deposit(uint256 amount) public {
        balances[msg.sender] += amount;
    }

    function withdraw(uint256 amount) public {
        require(balances[msg.sender] >= amount, "Insufficient balance");
        // Your code here
    }
}

Hint

Wrap the subtraction inside unchecked { ... } to save gas.

4

Print final contract code

Print the entire GasOptimizedL2 contract code to verify your final optimized contract.

Blockchain / Solidity

contract GasOptimizedL2 {
    mapping(address => uint256) public balances;

    function deposit(uint256 amount) public {
        balances[msg.sender] += amount;
    }

    function withdraw(uint256 amount) public {
        require(balances[msg.sender] >= amount, "Insufficient balance");
        unchecked {
            balances[msg.sender] -= amount;
        }
    }
}

// Your code here to print the contract code

Hint

Since Solidity contracts are not printed at runtime, confirm your final code matches the optimized contract.

Practice

1. Which of the following is a common method to reduce gas costs on Layer 2 blockchains?

easy

A. Using loops with many iterations

B. Increasing the block size limit

C. Adding more storage variables to the contract

D. Using calldata instead of memory for function inputs

Solution

1. Step 1: Understand calldata vs memory

   Calldata is cheaper than memory because it is read-only and does not require copying data.

2. Step 2: Identify gas saving method

   Using calldata for function inputs reduces gas compared to memory or storage.

3. Final Answer:

   Using calldata instead of memory for function inputs -> Option D

4. Quick Check:

   Calldata is cheaper than memory [OK]

Hint: Choose calldata for inputs to save gas on L2 [OK]

Common Mistakes:

• Thinking increasing block size reduces gas
• Assuming more storage variables save gas
• Using loops without optimization

2. Which Solidity syntax correctly declares a function parameter to use calldata for gas optimization on L2?

easy

A. function foo(string memory data) external {}

B. function foo(string calldata data) external {}

C. function foo(string storage data) external {}

D. function foo(string data) external {}

Solution

1. Step 1: Recall parameter data location keywords

   Solidity allows memory, storage, or calldata for reference types in parameters.

2. Step 2: Identify calldata usage

   Calldata is specified explicitly as string calldata for external functions to save gas.

3. Final Answer:

   function foo(string calldata data) external {} -> Option B

4. Quick Check:

   Calldata keyword used correctly [OK]

Hint: Use 'calldata' keyword for external function parameters [OK]

Common Mistakes:

• Using memory instead of calldata for external inputs
• Omitting data location keyword
• Using storage incorrectly in parameters

3. What will be the gas cost difference when using unchecked math in Solidity on L2 compared to normal math?

medium

A. Gas cost decreases because overflow checks are skipped

B. Gas cost increases due to extra checks

C. Gas cost stays the same

D. Gas cost is unpredictable

Solution

1. Step 1: Understand unchecked math

   Unchecked math skips overflow and underflow checks, saving gas.

2. Step 2: Compare gas costs

   Skipping checks reduces gas cost compared to normal safe math operations.

3. Final Answer:

   Gas cost decreases because overflow checks are skipped -> Option A

4. Quick Check:

   Unchecked math saves gas by skipping checks [OK]

Hint: Unchecked math skips checks, lowering gas [OK]

Common Mistakes:

• Assuming unchecked math adds overhead
• Believing gas cost is unchanged
• Ignoring overflow risks

4. Given this Solidity snippet on L2, what is the main issue causing higher gas usage?

uint256 public count;
function increment() external {
  count = count + 1;
}

medium

A. Missing unchecked block for increment

B. Using public variable instead of private

C. Function should be view instead of external

D. No issue, code is optimal

Solution

1. Step 1: Identify gas cost in arithmetic

   Normal addition includes overflow checks increasing gas.

2. Step 2: Suggest unchecked usage

   Wrapping increment in unchecked { count += 1; } saves gas by skipping checks.

3. Final Answer:

   Missing unchecked block for increment -> Option A

4. Quick Check:

   Unchecked block reduces gas for safe increments [OK]

Hint: Use unchecked for simple increments to save gas [OK]

Common Mistakes:

• Thinking public visibility affects gas here
• Confusing external with view function
• Assuming code is already optimized

5. You want to optimize a Layer 2 contract that stores multiple small variables. Which approach best reduces gas usage?

hard

A. Use dynamic arrays for all variables

B. Store each variable in separate storage slots for clarity

C. Pack multiple uint8 variables into a single uint256 storage slot

D. Avoid using calldata and always copy to memory

Solution

1. Step 1: Understand storage slot packing

   Multiple small variables like uint8 can fit into one 256-bit slot, saving gas.

2. Step 2: Compare storage strategies

   Separating variables wastes slots; dynamic arrays add overhead; calldata usage unrelated here.

3. Final Answer:

   Pack multiple uint8 variables into a single uint256 storage slot -> Option C

4. Quick Check:

   Storage packing reduces gas by slot sharing [OK]

Hint: Pack small vars into one slot to save gas [OK]

Common Mistakes:

• Using separate slots wastes gas
• Overusing dynamic arrays increases cost
• Ignoring calldata benefits in storage