使用 unchecked 有什么好处？

Question

在OpenZeppelin ERC20实现中，有一个_transfer方法：

function _transfer(
    address sender,
    address recipient,
    uint256 amount
) internal virtual {
    require(sender != address(0), "ERC20: transfer from the zero address");
    require(recipient != address(0), "ERC20: transfer to the zero address");

    _beforeTokenTransfer(sender, recipient, amount);

    uint256 senderBalance = _balances[sender];
    require(senderBalance >= amount, "ERC20: transfer amount exceeds balance");
    unchecked {
        _balances[sender] = senderBalance - amount;
    }
    _balances[recipient] += amount;

    emit Transfer(sender, recipient, amount);

    _afterTokenTransfer(sender, recipient, amount);
}

为什么他们使用未经检查的算术来减少余额？我知道，如果未经检查，2-3 将返回 2**256-1 并且不会出现异常。但为什么我们需要这个？

Answer 1

unchecked与常规算术运算相比，生成更小的字节码，因为它不包含下溢/溢出验证。

因此，如果您希望在发生溢出时有自定义错误消息，则此代码的运行成本较低

uint256 senderBalance = _balances[sender];
require(senderBalance >= amount, "ERC20: transfer amount exceeds balance");
unchecked {

    // no validation here as it's already validated in the `require()` condition
    _balances[sender] = senderBalance - amount;
}

与这个相比

uint256 senderBalance = _balances[sender];
require(senderBalance >= amount, "ERC20: transfer amount exceeds balance");

// redundant validation here
_balances[sender] = senderBalance - amount;

如果没有自定义消息，这将是最便宜但仍然安全的选项：

// contains the check and fails without custom message in case of underflow
_balances[sender] -= amount;

而且这比之前的还要便宜。但它是不安全的，因为它不检查下溢：

unchecked {
    // UNSAFE, DO NOT USE
    _balances[sender] -= amount;
}