Packing variables for gas savings in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is variable packing in blockchain smart contracts?

Variable packing is the technique of grouping multiple smaller variables into a single storage slot to reduce gas costs by minimizing storage usage.

beginner

Why does packing variables save gas in Ethereum smart contracts?

Because Ethereum charges gas based on storage slots used, packing multiple small variables into one 32-byte slot reduces the number of slots, lowering gas consumption.

intermediate

Which data types are best suited for packing together?

Smaller data types like uint8, bool, or bytes1 can be packed together efficiently since they take less than 32 bytes each and fit into one storage slot.

intermediate

What happens if variables are declared out of order for packing?

If variables are not ordered from smallest to largest, the compiler may not pack them efficiently, causing more storage slots to be used and increasing gas costs.

beginner

Show an example of packing two uint128 variables in Solidity.

```solidity contract Packed { uint128 a; uint128 b; } ``` Here, both uint128 variables fit into one 32-byte storage slot, saving gas.

Why is packing variables important in Ethereum smart contracts?

A To avoid compiler errors

B To increase contract execution speed

C To reduce gas costs by minimizing storage slots used

D To make code easier to read

Packing variables reduces the number of storage slots used, which lowers gas costs.

Which of these variable types can be packed together efficiently?

A uint256 and uint128

B uint8 and bool

C string and bytes

D address and uint256

Small types like uint8 and bool fit together in one storage slot, saving gas.

What is the size of one storage slot in Ethereum?

A 64 bytes

B 16 bytes

C 8 bytes

D 32 bytes

Each storage slot in Ethereum is 32 bytes.

What is a common mistake that reduces packing efficiency?

A Declaring variables in random order

B Using only uint8 variables

C Using constants

D Using public variables

Declaring variables out of size order can prevent efficient packing.

How many uint128 variables can fit in one storage slot?

A Two

B One

C Four

D Eight

Two uint128 variables (16 bytes each) fit into one 32-byte storage slot.

Explain how packing variables can reduce gas costs in Ethereum smart contracts.

Think about how Ethereum charges for storage and how combining variables helps.

Describe best practices for ordering variables to maximize packing efficiency.

Consider how variable size affects packing in storage slots.

Practice

1.

What is the main benefit of packing variables in blockchain smart contracts?

easy

A. It makes the code run faster on the CPU

B. It reduces gas costs by using fewer storage slots

C. It increases the contract's storage size

D. It improves the contract's security automatically

Solution

1. Step 1: Understand storage slots in blockchain

   Each storage slot costs gas. Packing variables reduces the number of slots used.

2. Step 2: Effect of packing variables

   By grouping smaller variables into one slot, gas usage decreases because fewer slots are needed.

3. Final Answer:

   It reduces gas costs by using fewer storage slots -> Option B

4. Quick Check:

   Packing = Less gas [OK]

Hint: Packing saves gas by reducing storage slots used [OK]

Common Mistakes:

• Thinking packing speeds up CPU execution
• Believing packing increases storage size
• Assuming packing improves security automatically

2.

Which of the following Solidity variable declarations correctly packs two uint8 variables into one storage slot?

contract Example {
    uint8 a;
    uint8 b;
}

easy

A. Both a and b are declared consecutively as uint8

B. a is uint8 and b is uint256

C. a is uint256 and b is uint8

D. Both a and b are declared as uint256

Solution

1. Step 1: Check variable types for packing

   Variables of the same small type declared consecutively can be packed into one slot.

2. Step 2: Analyze given declarations

   Both a and b are uint8, so they fit together in one 256-bit slot.

3. Final Answer:

   Both a and b are declared consecutively as uint8 -> Option A

4. Quick Check:

   Same small types together = packed [OK]

Hint: Use same small types consecutively to pack [OK]

Common Mistakes:

• Mixing large and small types breaks packing
• Declaring variables far apart prevents packing
• Assuming different types pack automatically

3.

Consider this Solidity contract snippet:

contract Test {
    uint128 a = 1;
    uint64 b = 2;
    uint64 c = 3;
}

How many storage slots does this contract use for these variables?

medium

A. 1 storage slot

B. 3 storage slots

C. 2 storage slots

D. 4 storage slots

Solution

1. Step 1: Calculate sizes of variables

   a is 128 bits, b and c are 64 bits each.

2. Step 2: Pack variables into 256-bit slots

   a (128 bits) starts the slot, b and c (64+64=128 bits) fit exactly in the remaining space of the same slot.

3. Final Answer:

   1 storage slot -> Option A

4. Quick Check:

   128 + 64 + 64 = 256 bits = 1 slot [OK]

Hint: Sum consecutive vars' bits: 128+64+64=256 -> 1 slot [OK]

Common Mistakes:

• Counting each variable as one slot
• Ignoring packing into remaining space after uint128
• Assuming different sizes cannot pack together

4.

Identify the error in this Solidity contract that tries to pack variables for gas savings:

contract Faulty {
    uint256 a;
    uint8 b;
    uint256 d;
    uint8 c;
}

medium

A. Variables b and c must be declared before a

B. Variables a and d should be uint8 to pack

C. All variables are packed correctly without error

D. Variables b and c cannot be packed because d breaks the sequence

Solution

1. Step 1: Understand packing order

   Packing works best when small variables are declared consecutively without large variables interrupting.

2. Step 2: Analyze variable order

   b and c are small but separated by large a and d, so they cannot be packed together.

3. Final Answer:

   Variables b and c cannot be packed because d breaks the sequence -> Option D

4. Quick Check:

   Interrupting large vars break packing [OK]

Hint: Keep small vars together without large vars interrupting [OK]

Common Mistakes:

• Assuming all small vars pack regardless of order
• Changing types unnecessarily
• Thinking order does not affect packing

5.

You want to optimize this Solidity struct for gas savings by packing variables:

struct Data {
    uint8 flag;
    uint256 id;
    uint16 count;
    uint8 status;
}

Which rearrangement packs variables best into the fewest storage slots?

hard

A. struct Data {
uint8 flag;
uint256 id;
uint16 count;
uint8 status;
}

B. struct Data {
uint16 count;
uint256 id;
uint8 flag;
uint8 status;
}

C. struct Data {
uint8 flag;
uint8 status;
uint16 count;
uint256 id;
}

D. struct Data {
uint8 flag;
uint16 count;
uint256 id;
uint8 status;
}

Solution

1. Step 1: Order variables from smallest to largest

   Place small variables (uint8, uint16) together first to pack tightly.

2. Step 2: Place largest variable last

   uint256 id is largest and should be last to avoid breaking packing of smaller vars.

3. Final Answer:

   struct Data { uint8 flag; uint8 status; uint16 count; uint256 id; } -> Option C

4. Quick Check:

   Small vars first, large last = best packing [OK]

Hint: Put small vars first, big vars last for best packing [OK]

Common Mistakes:

• Placing large vars before small ones
• Mixing variable sizes without order
• Ignoring packing benefits in structs