storage-layout

Purpose

Identify storage slot inefficiencies in Solidity contracts and recommend layout changes that reduce SSTORE and SLOAD costs. Storage operations are the most expensive EVM instructions: a cold SSTORE costs 22,100 gas and a cold SLOAD costs 2,100 gas. Every eliminated slot and every avoided redundant storage access compounds across all callers for the life of the contract.

This skill covers 10 techniques (SL-001 through SL-010) derived from the gas optimization knowledge base. It does not invent new rules.

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When to Use This Skill

• Writing or reviewing a struct definition with mixed-size fields
• Reviewing state variable declarations at the contract level
• Auditing functions that read or write the same storage variable multiple times
• Reviewing reentrancy guards (check for SSTORE-based lock patterns)
• Reviewing flash-loan or callback patterns that use persistent storage flags
• Reviewing constructors that push static data into storage arrays
• Reviewing contracts that use arrays for key-based lookup instead of mappings

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When NOT to Use This Skill

• Upgradeable contracts (proxy pattern): Never reorder existing state variables. Storage layout is immutable across upgrades. Only append at the end, using gaps.
• Frozen/deployed contracts: If the contract is already deployed and not upgradeable, storage layout cannot be changed. Report findings as informational only.
• Separately accessed fields: If struct fields are always read in separate transactions, packing into shared slots adds masking overhead without reducing SLOAD count.
• Chains without Cancun support: Do not recommend transient storage (SL-005, SL-006) on chains that do not yet support EIP-1153 (e.g., some L2s before their Cancun upgrade).
• Mutable large data: Do not recommend SSTORE2 (SL-007) for data that changes after deployment — bytecode is immutable.

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Rationalizations to Reject

Rationalization	Why It's Wrong	Required Action
"The struct looks fine as-is"	Unpacked slots cost 22,100 gas per write, silently	Run the packing check regardless of visual appearance
"It's only one extra slot"	At 10,000 tx/day, 1 extra slot = 221M gas/day wasted	Eliminate every eliminable slot
"I'll optimize later"	Storage layout cannot be changed after deployment without a migration	Fix during development, not after
"The compiler handles this"	Solidity packs within consecutive declarations only; cross-declaration gaps are not packed	Always verify with forge inspect

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Platform Detection

Before applying any recommendation, verify the environment:

# Solidity files present?
ls src/**/*.sol 2>/dev/null || echo "No .sol files found"

# Foundry project?
test -f foundry.toml && echo "Foundry detected" || echo "No foundry.toml"

# Hardhat project?
test -f hardhat.config.ts || test -f hardhat.config.js && echo "Hardhat detected"

# Solidity version (check pragma for transient storage eligibility)
grep -r "pragma solidity" src/ | head -5

# EVM target (for transient storage: requires cancun or later)
grep "evm_version" foundry.toml 2>/dev/null

# Inspect slot layout (requires forge)
forge inspect <ContractName> storageLayout --json 2>/dev/null

Transient storage (SL-005, SL-006) requires: Solidity ^0.8.24 and EVM target cancun. If either condition is unmet, skip those techniques and note the constraint.

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Quick Reference

Variable size table

Type	Bytes	Packs With
bool	1	uint8, other bools, bytes1
uint8 / int8	1	Other small types
uint16 / int16	2	Other small types
uint32 / int32	4	Other small types
uint64 / int64	8	Other small types
uint96 / int96	12	address (perfect 32-byte fill)
uint128 / int128	16	Another uint128
address	20	uint96 or smaller
uint256 / int256	32	Nothing — takes a full slot alone
bytes32	32	Nothing — takes a full slot alone

Decision tree

Is the data written once and read many times (>2 reads)?
  YES → Is it large (>64 bytes)?
          YES → SL-007: SSTORE2
          NO  → SL-009: constant if literal hash
  NO  → Is the state only needed within one transaction?
          YES → SL-005 or SL-006: transient storage
          NO  → Does a function read/write the same slot N>1 times?
                  YES → SL-003 (cache) + SL-008 (batch write)
                  NO  → Are struct fields sorted large→small?
                          NO  → SL-001: repack struct
                          YES → Does address + field fill exactly 32 bytes?
                                  NO  → Check for uint96 pairing: SL-002

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Workflow

Step 1 — Identify variables

• List all struct definitions and state variable declarations in scope
• Note each field's type and byte size using the table in §6
• Flag any struct where fields are not ordered large-first

Step 2 — Calculate current slot usage

• Count the number of storage slots each struct occupies
• Run forge inspect <Contract> storageLayout --json if forge is available
• Identify wasted bytes within each slot (slot bytes used vs. 32)

Step 3 — Compute optimal layout

• Reorder fields: uint256/bytes32 first, then descending size, small types last
• Check for address + uint96 perfect-pair opportunities (SL-002)
• Identify functions with multiple reads of the same variable (SL-003)
• Identify functions with multiple writes to the same variable (SL-008)
• Check reentrancy guards: are they using a storage slot? (SL-005)
• Check for within-transaction flags: could they use transient storage? (SL-006)
• Check for keccak256("literal") calls in function bodies (SL-009)
• Check for arrays used as key-based stores instead of mappings (SL-010)

Step 4 — Apply and annotate

• Apply struct reordering; add inline slot comments (// slot 0, // slot 1, bytes 0-15)
• Add // SL-001: packed or equivalent comment to each modified struct
• Verify with forge inspect after changes — confirm expected slot count
• Run forge snapshot --diff to measure the gas delta

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Output Format

Report each finding using this structure:

**[SEVERITY] Description — N wasted slots / savings summary**
**File:** path/to/Contract.sol, line N
**Technique:** SL-00X
**Estimated saving:** ~X gas per [write/read/call]
[before code block]
[after code block]
**Verification:** forge inspect / forge snapshot --diff

Concrete example finding

[HIGH] Unpacked storage struct — 2 wasted slots File: src/Vault.sol, line 14 Technique: SL-001 Estimated saving: ~44,200 gas per write (2 eliminated cold SSTOREs)

Before:

struct Position {
    uint128 amount;    // slot 0 (wastes 16 bytes)
    uint256 principal; // slot 1 (forces new slot)
    uint128 reward;    // slot 2 (wastes 16 bytes)
    address owner;     // slot 3 (wastes 12 bytes)
    bool active;       // slot 4 (wastes 31 bytes)
}
// 5 slots — 5 SSTOREs on first write

After:

struct Position {
    uint256 principal; // slot 0
    uint128 amount;    // slot 1, bytes 0-15
    uint128 reward;    // slot 1, bytes 16-31
    address owner;     // slot 2, bytes 0-19
    bool active;       // slot 2, byte 20
}
// 3 slots — 3 SSTOREs on first write (saves 44,200 gas cold)

Verification: forge inspect Vault storageLayout --json then forge snapshot --diff

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Anti-Hallucination / Stability Rules

• All gas figures in this skill are derived from docs/evm-gas-reference.md and resources/PATTERNS.md. Do not invent new numbers or cite figures not present in those files.
• Do not recommend transient storage on chains or compiler versions that do not support EIP-1153. Always check §5 Platform Detection first.
• Do not recommend SSTORE2 for mutable data. Bytecode is immutable.
• Do not reorder struct fields in upgradeable proxy contracts — state this constraint explicitly if the contract uses UUPSUpgradeable, Initializable, or storage gaps.
• If forge is not available, skip forge inspect steps and note the limitation.
• Report only findings that are present in the code under review. Do not add speculative "consider also..." findings outside the scope of the 10 techniques.

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Supporting Docs

Only read these files when explicitly needed — do not load all three by default:

File	Read only when…
resources/PATTERNS.md	You encounter an SL pattern (SL-004 through SL-010) not covered by the Quick Reference above, or need to verify an edge case
resources/CHECKLIST.md	Producing a formal /decipher-gas-optimizoor:analyze report and need to confirm completeness
resources/EXAMPLE_FINDING.md	Generating a report and need the exact output format for a multi-finding DeFi vault example
docs/evm-gas-reference.md	You need authoritative opcode costs (SSTORE, SLOAD, TSTORE) or slot packing rules to back a gas estimate