Gas optimization with storage in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is 'gas' in blockchain smart contracts?

Gas is the fee required to perform operations on a blockchain like Ethereum. It measures the computational effort needed to execute transactions or smart contracts.

beginner

Why is storage expensive in smart contracts?

Storing data on the blockchain uses permanent space and requires more gas because it affects all nodes. Reading and writing storage costs more gas than using temporary memory.

intermediate

How can packing variables reduce gas costs?

Packing multiple smaller variables into a single storage slot reduces the number of storage slots used, lowering gas costs because fewer storage operations are needed.

intermediate

What is the benefit of using 'calldata' instead of 'memory' for function parameters?

Using 'calldata' for external function parameters avoids copying data into memory, saving gas because calldata is read-only and cheaper to access.

beginner

Explain why minimizing storage writes saves gas.

Each write to storage costs a lot of gas. Minimizing writes by updating storage only when necessary or batching changes reduces gas consumption significantly.

Which operation generally costs the most gas in a smart contract?

A Writing data to storage

B Reading data from memory

C Calling a view function

D Using local variables

Writing data to storage is the most expensive operation because it permanently changes the blockchain state.

What does 'packing variables' mean in gas optimization?

A Combining multiple small variables into one storage slot

B Compressing data before storing

C Using fewer functions

D Storing variables in memory instead of storage

Packing variables means placing multiple small variables into a single 32-byte storage slot to reduce storage usage.

Why is 'calldata' cheaper than 'memory' for function inputs?

A Calldata uses less gas to write

B Calldata is stored off-chain

C Memory is slower to access

D Calldata is read-only and does not require copying

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

Which of these reduces gas cost when updating storage?

A Writing to storage multiple times separately

B Using global variables instead of local

C Batching multiple updates into one transaction

D Avoiding function modifiers

Batching updates reduces the number of storage writes, saving gas.

What is a common practice to save gas when dealing with boolean variables?

A Store booleans in memory only

B Pack multiple booleans into one byte

C Convert booleans to strings

D Use separate storage slots for each boolean

Packing multiple booleans into one byte reduces storage slots used and saves gas.

Describe three ways to optimize gas costs related to storage in smart contracts.

Think about how data is stored and accessed in smart contracts.

Explain why storage operations are more expensive than memory operations in blockchain smart contracts.

Consider the difference between temporary and permanent data.

Practice

1. Why is it important to optimize storage usage in blockchain smart contracts?

easy

A. Because storage is unlimited and free to use

B. Because storage variables are faster to access than memory variables

C. Because storage operations consume more gas, increasing transaction costs

D. Because storage variables do not affect gas costs

Solution

1. Step 1: Understand gas cost of storage

   Storage operations in blockchain smart contracts consume significant gas compared to memory or stack operations.

2. Step 2: Impact on transaction cost

   Higher gas consumption means higher transaction fees for users interacting with the contract.

3. Final Answer:

   Because storage operations consume more gas, increasing transaction costs -> Option C

4. Quick Check:

   Storage gas cost = higher fees [OK]

Hint: Remember: storage costs gas, so optimize to save money [OK]

Common Mistakes:

• Thinking storage is free
• Confusing storage with memory gas costs
• Assuming storage is faster than memory

2. Which of the following Solidity declarations helps reduce gas by optimizing storage usage?

easy

A. uint8 public smallNumber; uint8 public anotherSmallNumber;

B. constant uint256 public MAX_VALUE = 1000;

C. uint256 public data;

D. string public name;

Solution

1. Step 1: Identify constant usage

   Declaring variables as constant stores their value in bytecode, not storage, saving gas.

2. Step 2: Compare with other declarations

   Other options use storage variables which consume gas on read/write, unlike constants.

3. Final Answer:

   constant uint256 public MAX_VALUE = 1000; -> Option B

4. Quick Check:

   Constants reduce storage gas [OK]

Hint: Use constants to save storage gas costs [OK]

Common Mistakes:

• Assuming all variables cost the same gas
• Not knowing constants are stored differently
• Ignoring variable size impact

3. Consider this Solidity snippet:

contract Test {
    uint8 a = 1;
    uint8 b = 2;
    uint16 c = 3;
}

How many storage slots does this contract use?

medium

A. 1 storage slot

B. 2 storage slots

C. 3 storage slots

D. 4 storage slots

Solution

1. Step 1: Understand storage slot packing

   Solidity packs variables into 32-byte slots if their sizes fit together.

2. Step 2: Calculate combined size

   uint8 + uint8 + uint16 = 1 + 1 + 2 = 4 bytes, which fits into one 32-byte slot.

3. Final Answer:

   1 storage slot -> Option A

4. Quick Check:

   Small types packed = 1 slot [OK]

Hint: Pack small variables to fit one storage slot [OK]

Common Mistakes:

• Assuming each variable uses one slot
• Ignoring variable size packing
• Confusing bytes with slots

4. Identify the gas optimization mistake in this Solidity code:

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

medium

A. Using uint256 instead of uint8 for small counters

B. Not marking count as immutable

C. Using count = count + 1; instead of ++count;

D. Not using constant for count

Solution

1. Step 1: Analyze variable type choice

   Using uint256 for a small counter wastes storage space and gas.

2. Step 2: Suggest smaller type

   Using uint8 or smaller type packs better and reduces gas cost.

3. Final Answer:

   Using uint256 instead of uint8 for small counters -> Option A

4. Quick Check:

   Smaller types save gas [OK]

Hint: Use smallest uint type needed for counters [OK]

Common Mistakes:

• Thinking increment syntax affects gas
• Confusing constant and immutable usage
• Ignoring variable size impact

5. You want to store multiple boolean flags in a contract efficiently. Which approach optimizes gas usage best?

hard

A. Use separate bool variables for each flag

B. Use multiple uint256 variables for each flag

C. Use an array of bool variables

D. Use a single uint8 variable and bitwise operations to store flags

Solution

1. Step 1: Understand storage cost of booleans

   Each bool uses a full storage slot if declared separately, wasting gas.

2. Step 2: Use bit packing with uint8

   Storing multiple flags as bits in a single uint8 packs them efficiently, reducing storage slots and gas.

3. Step 3: Compare other options

   Arrays and multiple uint256 variables consume more storage and gas.

4. Final Answer:

   Use a single uint8 variable and bitwise operations to store flags -> Option D

5. Quick Check:

   Bit packing booleans saves gas [OK]

Hint: Pack booleans into bits using uint types [OK]

Common Mistakes:

• Using separate bools wastes storage
• Assuming arrays are cheaper
• Ignoring bitwise operations benefits