《纸上谈兵·solidity》第 21 课：Gas 优化与成本分析 —— 写出便宜的智能合约

// 差的写法
function bad(uint256 x) external {
    for (uint i = 0; i < 10; i++) {
        storageVar += x;
    }
}

// 优化写法
function good(uint256 x) external {
    uint256 tmp = storageVar;
    for (uint i = 0; i < 10; i++) {
        tmp += x;
    }
    storageVar = tmp;
}

// 差的写法
function sum(uint256[] memory arr) external pure returns (uint256 s) {
    for (uint i = 0; i < arr.length; i++) {
        s += arr[i];
    }
}

// 优化写法
function sum(uint256[] calldata arr) external pure returns (uint256 s) {
    for (uint i = 0; i < arr.length; i++) {
        s += arr[i];
    }
}

// 差的写法
struct Bad {
    uint128 a;
    uint128 b;
    uint128 c;
}

// 优化写法（两个 slot -> 一个 slot）
struct Good {
    uint128 a;
    uint128 b;
    uint256 c;
}

// 差的写法
event Deposit(address indexed user, uint256 amount, string memo);

// 优化写法
event Deposit(address indexed user, uint256 amount);

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract BadContract {
    uint256 public value;

    function addMany(uint256 x) external {
        for (uint i = 0; i < 10; i++) {
            value += x; // 每次都访问 storage
        }
    }
}

contract GoodContract {
    uint256 public value;

    function addMany(uint256 x) external {
        uint256 tmp = value;
        for (uint i = 0; i < 10; i++) {
            tmp += x;
        }
        value = tmp; // 只写一次 storage
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import "forge-std/Test.sol";
import "../src/Gas.sol";

contract GasTest is Test {
    BadContract bad;
    GoodContract good;

    function setUp() public {
        bad = new BadContract();
        good = new GoodContract();
    }

    function testGasBad() public {
        bad.addMany(1);
    }

    function testGasGood() public {
        good.addMany(1);
    }
}

➜  tutorial git:(main) ✗ forge test --match-path test/Gas.t.sol --gas-report -vvv 

[⠊] Compiling...
[⠒] Compiling 2 files with Solc 0.8.30
[⠑] Solc 0.8.30 finished in 506.84ms
Compiler run successful!

Ran 2 tests for test/Gas.t.sol:GasTest
[PASS] testGasBad() (gas: 53693)
[PASS] testGasGood() (gas: 51716)
Suite result: ok. 2 passed; 0 failed; 0 skipped; finished in 3.57ms (894.88µs CPU time)

╭----------------------------------+-----------------+-------+--------+-------+---------╮
| src/Gas.sol:BadContract Contract |                 |       |        |       |         |
+=======================================================================================+
| Deployment Cost                  | Deployment Size |       |        |       |         |
|----------------------------------+-----------------+-------+--------+-------+---------|
| 152487                           | 489             |       |        |       |         |
|----------------------------------+-----------------+-------+--------+-------+---------|
|                                  |                 |       |        |       |         |
|----------------------------------+-----------------+-------+--------+-------+---------|
| Function Name                    | Min             | Avg   | Median | Max   | # Calls |
|----------------------------------+-----------------+-------+--------+-------+---------|
| addMany                          | 48203           | 48203 | 48203  | 48203 | 1       |
╰----------------------------------+-----------------+-------+--------+-------+---------╯

╭-----------------------------------+-----------------+-------+--------+-------+---------╮
| src/Gas.sol:GoodContract Contract |                 |       |        |       |         |
+========================================================================================+
| Deployment Cost                   | Deployment Size |       |        |       |         |
|-----------------------------------+-----------------+-------+--------+-------+---------|
| 153147                            | 492             |       |        |       |         |
|-----------------------------------+-----------------+-------+--------+-------+---------|
|                                   |                 |       |        |       |         |
|-----------------------------------+-----------------+-------+--------+-------+---------|
| Function Name                     | Min             | Avg   | Median | Max   | # Calls |
|-----------------------------------+-----------------+-------+--------+-------+---------|
| addMany                           | 46257           | 46257 | 46257  | 46257 | 1       |
╰-----------------------------------+-----------------+-------+--------+-------+---------╯


Ran 1 test suite in 6.50ms (3.57ms CPU time): 2 tests passed, 0 failed, 0 skipped (2 total tests)