solidity-gas-optimizer

When

After all tests pass for a contract. Before security review or deployment. Also when the user asks about gas efficiency, "can we reduce costs?", or after any significant implementation change that might affect gas consumption.

The strict rule

NO DEPLOYMENT WITHOUT A GAS AUDIT REPORT IN docs/audits/

Hard Gate

This skill dispatches the optimizoor agent to run the full 8-category checklist. No deployment proceeds without:

1. The gas audit report at docs/audits/YYYY-MM-DD-<contract>-gas.md
2. forge snapshot --diff output showing before/after gas changes
3. Contract size verified under 24,576 bytes

Mandatory Checklist

Step 1: Capture baseline

forge snapshot
forge inspect <ContractName> storage-layout --pretty
forge build --sizes

Step 2: Dispatch optimizoor

When this skill is invoked:

1. Run forge snapshot to capture the pre-optimization baseline
2. Run forge inspect <ContractName> storage-layout --pretty for all contracts
3. Dispatch the optimizoor agent with:
   • Full source code of all in-scope contracts
   • Output of forge inspect storage-layout for each contract
   • Output of forge build --sizes
   • The .gas-snapshot baseline
4. Wait for the optimizoor report
5. For each finding: apply the fix, run forge test (must pass), run forge snapshot --diff
6. Write the final gas audit report

Step 3: Apply findings

For every finding the optimizoor returns:

• Apply the fix to the source file
• Run forge test — all tests must pass after the fix
• Run forge snapshot --diff — record the delta
• Note the gas saved in the audit report

Step 4: Write the gas audit report

Save to docs/audits/YYYY-MM-DD-<contract>-gas.md using the format in the Output Artifacts section below.

Gas Optimization Checklist (8 Categories)

See gas-checklist.md for full code examples, false-positive warnings, and the gas cost reference table.

Category 1: Storage Layout — Highest Impact (Architectural, Permanent)

1.1 Struct slot packing — Order fields so smaller types share slots.

// BAD: 3 slots
struct Position { uint128 amount; address owner; uint128 interest; }

// GOOD: 2 slots (amount + owner in slot 0)
struct Position { uint128 amount; address owner; uint128 interest; }
// amount (16B) + owner (20B) = 36B → doesn't fit. Reorder:
struct Position { address owner; uint128 amount; uint128 interest; }
// owner (20B) + amount (12B from uint96) → use uint96:
struct Position { address owner; uint96 amount; uint64 interest; } // 1 slot

Rule: order struct fields from largest to smallest type within each slot group. Group fields that are read together in the same slot.

1.2 constant for compile-time values — Zero storage cost, zero SLOAD.

// BAD: SLOAD on every read (100–2100 gas)
uint256 public MAX_FEE = 1000;

// GOOD: inlined at compile time (0 gas to read)
uint256 public constant MAX_FEE = 1000;

Applies to: fee caps, role constants (bytes32 hashes), magic numbers, immutable protocol addresses.

1.3 immutable for constructor-set values — Stored in bytecode, costs 3 gas to read.

// BAD: SLOAD per read (100–2100 gas)
address public asset;
constructor(address _asset) { asset = _asset; }

// GOOD: bytecode storage, 3 gas to read
address public immutable asset;
constructor(address _asset) { asset = _asset; }

Note: immutable cannot be used with upgradeable contracts (no constructor). Use constant where possible; for upgradeable, initialize to storage once in initialize().

1.4 Boolean packing into bitmap — 5+ booleans should use a single uint256 bitmap.

// BAD: 5 storage slots
bool public isActive;
bool public isPaused;
bool public isDepositEnabled;
bool public isWithdrawEnabled;
bool public isEmergencyMode;

// GOOD: 1 storage slot
uint256 private _flags;
uint256 private constant FLAG_ACTIVE           = 1 << 0;
uint256 private constant FLAG_PAUSED           = 1 << 1;
uint256 private constant FLAG_DEPOSIT_ENABLED  = 1 << 2;
uint256 private constant FLAG_WITHDRAW_ENABLED = 1 << 3;
uint256 private constant FLAG_EMERGENCY        = 1 << 4;

function isActive() public view returns (bool) { return _flags & FLAG_ACTIVE != 0; }

Category 2: Function Visibility

2.1 external vs public for non-internally-called functions

public copies calldata to memory. external reads directly from calldata.

// BAD: copies calldata to memory
function processOrders(Order[] memory orders) public { ... }

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

Rule: if a function is never called internally, make it external.

2.2 Remove explicit getters for public state variables

Solidity auto-generates getters for all public state variables.

// BAD: manual getter duplicates auto-generated one (wastes deployment gas)
uint256 public fee;
function getFee() external view returns (uint256) { return fee; } // DELETE THIS

// GOOD: public generates getFee() automatically
uint256 public fee;

Category 3: Calldata vs Memory

3.1 External function array/struct parameters: calldata over memory

// BAD: memory copies the array
function batchTransfer(address[] memory recipients, uint256[] memory amounts) external { ... }

// GOOD: calldata avoids the copy (scales with array size)
function batchTransfer(address[] calldata recipients, uint256[] calldata amounts) external { ... }

3.2 Use uint256 for computation-only parameters

// BAD: uint8 in computation-only param causes masking overhead
function compute(uint8 multiplier) external pure returns (uint256) { ... }

// GOOD: uint256 for computation; small types only for storage
function compute(uint256 multiplier) external pure returns (uint256) { ... }

Exception: small types are correct for storage struct fields — packing is more important there.

Category 4: Loop Optimization

4.1 Cache array length before loop

// BAD: .length is SLOAD on every iteration for storage arrays
for (uint256 i; i < recipients.length; ++i) { ... }

// GOOD: cache in memory (free MLOAD after first read)
uint256 len = recipients.length;
for (uint256 i; i < len; ++i) { ... }

4.2 Cache storage reads inside loops

// BAD: SLOAD on every iteration (100–2100 gas each)
for (uint256 i; i < len; ++i) { total += balances[user]; }

// GOOD: 1 SLOAD, then MLOAD (3 gas) per iteration
uint256 userBalance = balances[user];
for (uint256 i; i < len; ++i) { total += userBalance; }

4.3 ++i over i++ — i++ creates a temporary copy. ++i modifies in place. ~5 gas/iteration.

4.4 unchecked loop counters — Counters bounded by array length cannot overflow.

// BAD: Solidity 0.8 overflow check on every increment (~30 gas wasted)
for (uint256 i; i < len; ++i) { ... }

// GOOD: unchecked counter saves ~30 gas/iteration
for (uint256 i; i < len; ) {
    // loop body
    unchecked { ++i; } // overflow impossible: i bounded by len, len bounded by gas limit
}

Category 5: Arithmetic — Unchecked Blocks

Use unchecked only when overflow is provably impossible. The proving comment is mandatory.

// GOOD: unchecked subtraction after comparison
function safeDecrement(uint256 balance, uint256 amount) internal pure returns (uint256) {
    if (balance < amount) revert InsufficientBalance(balance, amount);
    // Safe: comparison above proves balance >= amount, so underflow impossible
    unchecked { return balance - amount; }
}

// BAD: no proof, no comment — do not write this
unchecked { return a - b; }

Precision rule: never divide before multiplying.

// BAD: (a / b) * c — division truncates first, loses precision
uint256 result = (principal / totalSupply) * newRate;

// GOOD: (a * c) / b — multiply first, then divide
uint256 result = (principal * newRate) / totalSupply;

Category 6: Custom Errors

String reverts cost ~50 gas per character. Custom errors use 4-byte selectors.

// BAD: ~1600 gas for the string storage
require(condition, "InsufficientBalance: amount exceeds balance");

// GOOD: ~50 gas for the 4-byte selector
error InsufficientBalance(uint256 requested, uint256 available);
if (!condition) revert InsufficientBalance(requested, available);

The optimizoor agent must flag every require(condition, "string"). There should be zero.

Category 7: Compiler Configuration

7.1 optimizer_runs tuning

# foundry.toml

# Deployed once, called rarely (factories, one-time setup)
[profile.default]
optimizer = true
optimizer_runs = 200

# Called frequently (AMM, vault, token) — optimize for cheap calls over cheap deploy
[profile.production]
optimizer = true
optimizer_runs = 10000

7.2 via_ir for complex contracts

via_ir = true

Warning: via_ir increases compile time ~3x. Use when the contract has complex control flow or optimizer output from the normal pipeline is worse than expected. Always compare snapshots before and after enabling.

7.3 Solidity version — pin to latest stable

pragma solidity 0.8.25; // pin exact version, never floating (^0.8.x)

Solidity 0.8.22+ includes loop overflow checks the compiler can sometimes eliminate automatically.

Category 8: Events vs Storage

8.1 Off-chain-only data → events

// BAD: 20,000+ gas SSTORE for data only an indexer reads
mapping(address => uint256) public depositTimestamps;
depositTimestamps[msg.sender] = block.timestamp;

// GOOD: ~375 gas LOG for off-chain data
event DepositTimestamp(address indexed user, uint256 timestamp);
emit DepositTimestamp(msg.sender, block.timestamp);

8.2 Indexed parameters: only for filtered fields

Each indexed parameter costs 375 extra gas. Index only fields off-chain systems filter by.

// BAD: amounts rarely filtered — wastes 375 gas indexing them
event Transfer(address indexed from, address indexed to, uint256 indexed amount);

// GOOD: index only filterable fields
event Transfer(address indexed from, address indexed to, uint256 amount);

Forge Commands

# Inspect storage layout — do this FIRST
forge inspect <ContractName> storage-layout --pretty

# Create gas baseline
forge snapshot

# Compare after optimizations
forge snapshot --diff

# Detailed gas per function per test
forge test --gas-report

# Check contract bytecode size (max: 24,576 bytes)
forge build --sizes

# Measure gas with a specific Foundry profile
FOUNDRY_PROFILE=production forge test --gas-report

Output Artifacts

Gas audit report at docs/audits/YYYY-MM-DD-<contract>-gas.md:

# Gas Audit Report: <ContractName>
**Date:** YYYY-MM-DD
**Auditor:** optimizoor agent
**Baseline snapshot:** <attach forge snapshot output>

## Findings Summary
| ID | Category | Severity | Gas Saved | File:Line |
|----|----------|----------|-----------|-----------|
| G-01 | Storage Layout | HIGH | ~15,000/tx | src/Vault.sol:47 |
| G-02 | Loop | MEDIUM | ~30/iteration | src/Vault.sol:112 |

## Finding Detail

### G-01: Struct fields not packed — 3 slots used instead of 2
**File:** src/Vault.sol, lines 45-52
**Current code:**
struct Position { uint128 amount; address owner; uint128 interest; }
**Recommended fix:**
struct Position { address owner; uint128 amount; uint128 interest; }
**Estimated savings:** 1 SLOAD per read = 100-2100 gas per read
**Risk:** None — pure reordering, no behavioral change

## forge snapshot diff
<attach output of forge snapshot --diff>

## Contract sizes
<attach output of forge build --sizes>

Terminal State

Exit to solidity-code-reviewer (security review) or solidity-deployer (if review is done).

Blocked Rationalizations

Rationalization	Counter
"Gas optimization is premature"	Storage layout and immutable/constant decisions are architectural. They cannot be changed after deployment without a migration. Premature optimization is micro-optimizing; architecture is mandatory.
"The optimizer handles it"	The Solidity optimizer doesn't reorder struct fields, choose between constant and storage variables, or decide between events and storage writes. You do.
"This is a testnet deploy"	Testnet deploys become mainnet patterns. If the struct is packed wrong on testnet, it will be packed wrong on mainnet. Fix it now.
"The savings are too small to matter"	1,000 gas × 100,000 calls/year × 20 gwei × $2,000/ETH = $4,000/year in user fees. It matters.
"The contract is simple"	Simple contracts often have the easiest wins: a single immutable conversion, a struct reorder, one loop fix. Low-hanging fruit with no risk.