在此 OpenZeppelin ERC20 实现中声明一个额外的局部变量的目的是什么？

Question

从使用solidity ^0.8.0的Openzeppelin的ERC20实现来看，_transfer函数是这样的：

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");
        _balances[sender] = senderBalance - amount;
        _balances[recipient] += amount;

        emit Transfer(sender, recipient, amount);
    }

写有没有特殊用途

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

而不是简单地

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

?

Answer 1

从 pull request comment that contains this commit 添加 senderBalance:

The addition of revert messages back into the 0.8 branch (#2491) generated double reading of some storage slot. This PR remove the double sload, which reduces gas costs of running the affected function.

所以这是因为降低了 gas 使用量。

“一次从存储读取，一次从内存读取”比“两次从存储读取”消耗更少的 gas。

在此 OpenZeppelin ERC20 实现中声明一个额外的局部变量的目的是什么？

What is the purpose of declaring an extra local variable in this OpenZeppelin ERC20 implementation?

ethereum

solidity