https://solidity-by-example.org/function-selector/

When a function is called, the first 4 bytes of 

calldata
specifies which function to call.

This 4 bytes is called a function selector.

Take for example, this code below. It uses 

call
to execute 
transfer
on a contract at the address 
addr
.

调用函数时，前 4 个字节 

calldata
指定要调用的函数。

这 4 个字节称为函数选择器。

The first 4 bytes returned from 

abi.encodeWithSignature(....)
is the function selector.

Perhaps you can save a tiny amount of gas if you precompute and inline the function selector in your code?

Here is how the function selector is computed.

例如，下面的这段代码。 它用 

call
执行 
transfer
在地址的合同上 
addr
.

addr.call(abi.encodeWithSignature("transfer(address,uint256)", 0xSomeAddress, 123))

从返回的前 4 个字节 

abi.encodeWithSignature(....)
是功能选择器。

如果您在代码中预先计算并内联函数选择器，也许您可​​以节省少量气体？

以下是函数选择器的计算方式。

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;

contract FunctionSelector {
    /*
    "transfer(address,uint256)"
    0xa9059cbb
    "transferFrom(address,address,uint256)"
    0x23b872dd
    */
    function getSelector(string calldata _func) external pure returns (bytes4) {
        return bytes4(keccak256(bytes(_func)));
    }
}