如何尽快实施strlen [英] How to implement strlen as fast as possible

问题描述

假设您正在使用x86 32位系统。

Assume that you're working a x86 32-bits system. Your task is to implement the strlen as fast as possible.

您需要注意两个问题：
1.地址对齐。
2.读取具有机器字长（4个字节）的内存。

There're two problems you've to take care: 1. address alignment. 2. read memory with machine word length(4 bytes).

在给定的字符串中找到第一个对齐地址并不难。

It's not hard to find the first alignment address in the given string.

然后，我们可以用4个字节读取一次内存，并计算其总长度。但是，一旦4个字节中有一个零字节，我们就应该停止，并在零字节之前算出剩余字节。为了快速检查零字节，glibc提供了一个代码段：

Then we can read memory once with the 4 bytes, and count up it the total length. But we should stop once there's a zero byte in the 4 bytes, and count the left bytes before zero byte. In order to check the zero byte in a fast way, there's a code snippet from glibc:

unsigned long int longword, himagic, lomagic;
himagic = 0x80808080L;  
lomagic = 0x01010101L;

// There's zero byte in 4 bytes.
if (((longword - lomagic) & ~longword & himagic) != 0) {
    // do left thing...
}

我在Visual C ++中使用它来与CRT的实现进行比较。 CRT的速度比上述CRT快得多。

I used it in Visual C++, to compare with CRT's implementation. The CRT's is much more faster than the above one.

我对CRT的实现并不熟悉，他们是否使用更快的方法来检查零字节？

I'm not familiar with CRT's implementation, did they use a faster way to check the zero byte?

推荐答案

第一个CRT的是直接直接用汇编器编写的。您可以在此处查看其源代码 C：\Program Files\Microsoft Visual Studio 9.0\VC\crt\src\intel\strlen.asm （这是VS 2008的版本）

First CRT's one is written directly in assembler. you can see it's source code here C:\Program Files\Microsoft Visual Studio 9.0\VC\crt\src\intel\strlen.asm (this is for VS 2008)

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