Solidity Gas Optimization

Overview

Comprehensive gas optimization guide for Solidity smart contracts, containing 80+ techniques across 8 categories. Based on the RareSkills Book of Gas Optimization. Rules are prioritized by impact and safety.

When to Apply

Reference these guidelines when:

• Writing new Solidity smart contracts
• Reviewing or auditing existing contracts
• Optimizing gas costs for deployment or execution
• Refactoring contract storage layouts
• Implementing cross-contract interactions
• Choosing between design patterns (ERC721 vs ERC1155, etc.)

Priority-Ordered Categories

Priority	Category	Impact	Risk
1	Storage Optimization	CRITICAL	LOW
2	Deployment Optimization	HIGH	LOW
3	Calldata Optimization	HIGH	LOW
4	Design Patterns	HIGH	MEDIUM
5	Cross-Contract Calls	MEDIUM-HIGH	MEDIUM
6	Compiler Optimizations	MEDIUM	LOW
7	Assembly Tricks	MEDIUM	HIGH
8	Dangerous Techniques	LOW	CRITICAL

Quick Reference

Critical: Storage Optimization (Apply First)

Zero-to-One Writes:

• Avoid zero-to-one storage writes (costs 22,100 gas)
• Use 1/2 instead of 0/1 for boolean-like values
• Keep minimum balances in ERC20 contracts

Variable Packing:

// Bad: 3 slots
struct Unpacked {
    uint64 time;      // slot 1
    uint256 amount;   // slot 2
    address user;     // slot 3
}

// Good: 2 slots
struct Packed {
    uint64 time;      // slot 1 (with address)
    address user;     // slot 1
    uint256 amount;   // slot 2
}

Caching:

// Bad: reads storage twice
function increment() public {
    require(count < 10);
    count = count + 1;
}

// Good: reads storage once
function increment() public {
    uint256 _count = count;
    require(_count < 10);
    count = _count + 1;
}

Constants & Immutables:

uint256 constant MAX = 100;        // No storage slot
address immutable owner;           // Set in constructor, no storage

High: Deployment Optimization

Custom Errors:

// Bad: ~64+ bytes
require(amount <= limit, "Amount exceeds limit");

// Good: ~4 bytes
error ExceedsLimit();
if (amount > limit) revert ExceedsLimit();

Payable Constructors:

// Saves ~200 gas on deployment
constructor() payable {}

Clone Patterns:

• Use EIP-1167 minimal proxies for repeated deployments
• Use UUPS over Transparent Proxy for upgradeable contracts

High: Calldata Optimization

Calldata vs Memory:

// Bad: copies to memory
function process(bytes memory data) external {}

// Good: reads directly from calldata
function process(bytes calldata data) external {}

Avoid Signed Integers:

• Small negative numbers are expensive (e.g., -1 = 0xffff...)
• Use unsigned integers in function parameters

High: Design Patterns

Token Standards:

• Prefer ERC1155 over ERC721 for NFTs (no balanceOf overhead)
• Consider consolidating multiple ERC20s into one ERC1155

Signature vs Merkle:

• Prefer ECDSA signatures over Merkle trees for allowlists
• Implement ERC20Permit for approve + transfer in one tx

Alternative Libraries:

• Consider Solmate/Solady over OpenZeppelin for gas efficiency

Medium-High: Cross-Contract Calls

Reduce Interactions:

• Use ERC1363 transferAndCall instead of approve + transferFrom
• Implement multicall for batching operations
• Cache external call results (e.g., Chainlink oracles)

Access Lists:

• Use ERC2930 access list transactions to pre-warm storage

Medium: Compiler Optimizations

Loop Patterns:

// Good: unchecked increment, cached length
uint256 len = arr.length;
for (uint256 i; i < len; ) {
    // logic
    unchecked { ++i; }
}

Named Returns:

// More efficient bytecode
function calc(uint256 x) pure returns (uint256 result) {
    result = x * 2;
}

Bitshifting:

// Cheaper: 3 gas
x << 1   // x * 2
x >> 2   // x / 4

// Expensive: 5 gas
x * 2
x / 4

Short-Circuit Booleans:

• Place likely-to-fail conditions first in &&
• Place likely-to-succeed conditions first in ||

Medium: Assembly (Use Carefully)

Efficient Checks:

// Check address(0) with assembly
assembly {
    if iszero(caller()) { revert(0, 0) }
}

// Even/odd check
x & 1  // instead of x % 2

Memory Reuse:

• Reuse scratch space (0x00-0x40) for small operations
• Avoid memory expansion in loops

Avoid: Dangerous Techniques

These are unsafe for production:

• Making all functions payable
• Ignoring send() return values
• Using gasleft() for branching
• Manipulating block.number in tests

Outdated Patterns

These no longer apply in modern Solidity:

• "external is cheaper than public" - No longer true
• "!= 0 is cheaper than > 0" - Changed around 0.8.12

References

Full documentation with code examples:

• references/solidity-gas-guidelines.md - Complete guide
• references/rules/ - Individual patterns by category

To look up specific patterns:

grep -l "storage" references/rules/
grep -l "assembly" references/rules/
grep -l "struct" references/rules/

Rule Categories in references/rules/

• storage-* - Storage optimization patterns
• deploy-* - Deployment gas savings
• calldata-* - Calldata optimization
• design-* - Design pattern choices
• crosscall-* - Cross-contract call optimization
• compiler-* - Compiler optimization patterns
• assembly-* - Low-level assembly tricks

Key Principles

1. Always Benchmark - Compiler behavior varies by context and version
2. Balance Readability - Not all optimizations are worth code complexity
3. Test Both Approaches - Counterintuitive optimizations sometimes increase costs
4. Consider --via-ir - Modern compiler option may obsolete some tricks
5. Use Alternative Libraries - Solmate/Solady often beat OpenZeppelin on gas