57803 sc insight gas optimize paymentsinfo struct layout to save storage slots and reduce gas costs

Submitted on Oct 29th 2025 at 00:26:23 UTC by @chief_hunter888 for Audit Comp | Belong

• Report ID: #57803

• Report Type: Smart Contract

• Report severity: Insight

• Target: https://github.com/immunefi-team/audit-comp-belong/blob/main/contracts/v2/platform/BelongCheckIn.sol

Description

Optimize PaymentsInfo Struct Layout to Save Storage Slots and Reduce Gas Costs

Summary

There is a storage optimization for the PaymentsInfo struct inside BelongCheckIn.sol that saves one storage slot (approximately 20,000 gas per initialization) by reordering the fields inside the struct to yield more efficient storage slot usage. Reordering the fields slightly yields better gas/storage packing.

Current and proposed layouts, gas impact, and a proof-of-concept test are included below.

Current Struct Layout (UNOPTIMIZED) — 8 Slots

struct PaymentsInfo {
    uint96 slippageBps;           // Slot 0: 12 bytes
    uint24 swapPoolFees;          // Slot 0: + 3 bytes (15 total, 17 wasted)
    address swapV3Factory;        // Slot 1: 20 bytes (12 wasted)
    address swapV3Router;         // Slot 2: 20 bytes (12 wasted)
    address swapV3Quoter;         // Slot 3: 20 bytes (12 wasted)
    address wNativeCurrency;      // Slot 4: 20 bytes (12 wasted)
    address usdc;                 // Slot 5: 20 bytes (12 wasted)
    address long;                 // Slot 6: 20 bytes (12 wasted)
    uint256 maxPriceFeedDelay;    // Slot 7: 32 bytes
}

Total: 8 storage slots

Storage layout breakdown:

• Slot 0: uint96 (12 bytes) + uint24 (3 bytes) = 15 bytes used, 17 bytes wasted

• Slot 1–6: Each address (20 bytes) = 12 bytes wasted per slot

• Slot 7: uint256 (32 bytes) = fully utilized

Optimized Layout (PROPOSED) — 7 Slots

struct PaymentsInfo {
    address swapV3Factory;
    uint96 slippageBps;
    address swapV3Router;
    uint24 swapPoolFees;
    address swapV3Quoter;
    address wNativeCurrency;
    address usdc;
    address long;
    uint256 maxPriceFeedDelay;
}

Total: 7 storage slots

Storage layout breakdown:

• Slot 0: address (20 bytes) + uint96 (12 bytes) = 32 bytes (full)

• Slot 1: address (20 bytes) + uint24 (3 bytes) = 23 bytes used, 9 bytes wasted

• Slot 2–5: Each address (20 bytes) = 12 bytes wasted per slot

• Slot 6: uint256 (32 bytes) = fully utilized

Gas Savings

Cold storage write (first initialization)

• Unoptimized: ~160,000 gas (8 slots × ~20,000 gas/slot)

• Optimized: ~140,000 gas (7 slots × ~20,000 gas/slot)

• Savings: ~20,000 gas (~12.5% reduction)

Warm storage updates

• Similar cost for both layouts when updating individual fields; main savings come from initial deployment/initialization.

Cost per slot:

• Cold storage write (SSTORE): ~20,000 gas

• Warm storage write: ~5,000 gas

• Saving one slot = significant gas savings

Why This Matters

1. Cold storage writes occur when PaymentsInfo is initialized (e.g., contract deployment or setting payment configuration). Saving one slot saves ~20,000 gas.

2. Solidity packs variables into 32-byte storage slots. Placing smaller types (uint96, uint24) adjacent to address types (20 bytes) maximizes slot utilization.

3. The recommendation reduces the number of cold SSTOREs by one at initialization.

Recommendation

Apply this optimization to PaymentsInfo in:

• contracts/v2/platform/BelongCheckIn.sol (line 201)

Change from:

struct PaymentsInfo {
    uint96 slippageBps;
    uint24 swapPoolFees;
    address swapV3Factory;
    address swapV3Router;
    address swapV3Quoter;
    address wNativeCurrency;
    address usdc;
    address long;
    uint256 maxPriceFeedDelay;
}

Change to:

struct PaymentsInfo {
    address swapV3Factory;
    uint96 slippageBps;
    address swapV3Router;
    uint24 swapPoolFees;
    address swapV3Quoter;
    address wNativeCurrency;
    address usdc;
    address long;
    uint256 maxPriceFeedDelay;
}

Notes:

• No functional changes — only field order is changed.

• Data integrity preserved — fields store and retrieve correctly.

• Backward compatibility: existing code continues to work, but if deployed already, a storage migration is needed to avoid corrupting stored data.

How to Run the POC

1

1. Run the gas comparison test

npx hardhat test test/v2/platform/poc-storage-efficiency.test.ts

This runs the included proof-of-concept test that writes the unoptimized struct (cold write) and the optimized struct on a fresh contract and compares gas usage.

2

2. Expected output

See the expandable block below for the expected CLI output.

Expected Output

📊 UNOPTIMIZED STRUCT (Current Layout)
⛽ Gas Used (Cold Write): ~160,000 gas

📊 OPTIMIZED STRUCT (Proposed Layout)  
⛽ Gas Used (Cold Write): ~140,000 gas

💰 GAS SAVINGS COMPARISON
🎯 Gas Saved: ~20,000 gas
📉 Percentage Saved: ~12.5%

Proof of Concept (Test)

Add file to test/v3/platform/poc-storage-efficiency.test.ts and run using:

npx hardhat test test/v2/platform/poc-storage-efficiency.test.ts

POC test code:

import { ethers } from 'hardhat';
import { expect } from 'chai';
import { StorageEfficiencyTest } from '../../../typechain-types';
import { BigNumber } from 'ethers';


describe('Storage Efficiency POC - PaymentsInfo Struct Optimization', () => {
  let storageTest: StorageEfficiencyTest;
  let testAddresses: string[];

  // Sample data for testing
  const sampleData = {
    slippageBps: 500, // 5% slippage
    swapPoolFees: 3000, // 0.3% fee tier
    maxPriceFeedDelay: 3600, // 1 hour
  };

  beforeEach(async () => {
    const [owner, addr1, addr2, addr3, addr4, addr5, addr6] = await ethers.getSigners();
    
    // Deploy the storage test contract
    const StorageEfficiencyTestFactory = await ethers.getContractFactory('StorageEfficiencyTest');
    storageTest = await StorageEfficiencyTestFactory.deploy() as StorageEfficiencyTest;
    await storageTest.deployed();

    // Prepare test addresses
    testAddresses = [addr1.address, addr2.address, addr3.address, addr4.address, addr5.address, addr6.address];
  });

  describe('Gas Savings Comparison', () => {
    it('should compare and calculate gas savings from unoptimized to optimized struct', async () => {
      // Write to unoptimized struct
      const unoptimizedTx = await storageTest.writeUnoptimized(
        sampleData.slippageBps,
        sampleData.swapPoolFees,
        testAddresses[0],
        testAddresses[1],
        testAddresses[2],
        testAddresses[3],
        testAddresses[4],
        testAddresses[5],
        sampleData.maxPriceFeedDelay
      );
      const unoptimizedReceipt = await unoptimizedTx.wait();
      const unoptimizedGas = unoptimizedReceipt.gasUsed.toNumber();

      // Deploy a fresh contract for the optimized test (to ensure cold storage)
      const StorageEfficiencyTestFactory = await ethers.getContractFactory('StorageEfficiencyTest');
      const freshStorageTest = await StorageEfficiencyTestFactory.deploy() as StorageEfficiencyTest;
      await freshStorageTest.deployed();

      // Write to optimized struct
      const optimizedTx = await freshStorageTest.writeOptimized(
        sampleData.slippageBps,
        sampleData.swapPoolFees,
        testAddresses[0],
        testAddresses[1],
        testAddresses[2],
        testAddresses[3],
        testAddresses[4],
        testAddresses[5],
        sampleData.maxPriceFeedDelay
      );
      const optimizedReceipt = await optimizedTx.wait();
      const optimizedGas = optimizedReceipt.gasUsed.toNumber();

      const gasSaved = unoptimizedGas - optimizedGas;
      const percentageSaved = ((gasSaved / unoptimizedGas) * 100).toFixed(2);

      // Assert that we're saving gas
      expect(optimizedGas).to.be.lessThan(unoptimizedGas);
      // We expect to save approximately one storage slot worth of gas (~20,000 gas)
      expect(gasSaved).to.be.greaterThan(15000);
    });
  });
});

References

• Solidity Documentation: Layout of State Variables in Storage

• EIP-2200: Gas costs for storage operations

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If this change is applied to a deployed contract, perform a careful storage migration to avoid data corruption.