gas-optimization

Gas Optimization

Measure First

Never optimize blindly. Use forge snapshots and gas reports.

# Gas report for all tests
forge test --gas-report

# Snapshot to compare before/after
forge snapshot
# ...make changes...
forge snapshot --diff

Custom Errors Over Require Strings

Custom errors save ~50 gas on deployment and ~200 gas per revert vs require("string").

// BAD: ~24,000 gas on revert (stores string in bytecode + memory)
require(amount > 0, "Amount must be greater than zero");

// GOOD: ~24 gas for error selector encoding
error ZeroAmount();
if (amount == 0) revert ZeroAmount();

Calldata vs Memory

Use calldata for read-only external function parameters. Avoids copying to memory.

// BAD: copies entire array to memory (~3 gas per byte + allocation)
function processOrders(Order[] memory orders) external { ... }

// GOOD: reads directly from calldata
function processOrders(Order[] calldata orders) external { ... }

// Gas savings scale with input size:
// 100 bytes: ~600 gas saved
// 1KB:       ~6,000 gas saved
// 10KB:      ~60,000 gas saved

Immutable and Constant

// constant: compile-time value, inlined everywhere. 0 gas for SLOAD.
uint256 public constant MAX_SUPPLY = 10_000;
bytes32 public constant DOMAIN_TYPEHASH = keccak256("EIP712Domain(string name)");

// immutable: set once in constructor, stored in bytecode. 0 gas for SLOAD.
address public immutable FACTORY;
uint256 public immutable DEPLOYMENT_TIMESTAMP;

constructor(address factory) {
    FACTORY = factory;
    DEPLOYMENT_TIMESTAMP = block.timestamp;
}

// Regular storage variable: 2,100 gas (cold) or 100 gas (warm) per SLOAD
address public admin; // avoid if value never changes after deployment

Unchecked Arithmetic

When overflow is impossible (e.g., loop counters bounded by array length), use unchecked to skip overflow checks (~40 gas per operation).

// BAD: overflow checks on every increment
for (uint256 i = 0; i < arr.length; i++) { ... }

// GOOD: i cannot overflow (bounded by array length, which fits in uint256)
for (uint256 i; i < arr.length;) {
    // process arr[i]
    unchecked { ++i; }
}