什么是找到在C中的整数集最高位（MSB）最快/最有效的方法是什么？ [英] What is the fastest/most efficient way to find the highest set bit (msb) in an integer in C?

问题描述

如果我有一些整数n，我想知道最显著位的位置（即，如果最低显著位就在右边，我想知道最左侧位是一个位置1），什么是查不到的最快/最有效的方法？

If I have some integer n, and I want to know the position of the most significant bit (that is, if the least significant bit is on the right, I want to know the position of the furthest left bit that is a 1), what is the quickest/most efficient method of finding out?

我知道，POSIX支持strings.h一个 FFS（）方法寻找第一个设置位，但似乎并没有得到相应的 FLS（）方法。

I know that POSIX supports a ffs() method in strings.h to find the first set bit, but there doesn't seem to be a corresponding fls() method.

是否有这样做，我很想念的一些非常明显的方式？

Is there some really obvious way of doing this that I'm missing?

，你不能用怎么样的情况下POSIX功能的便携性？

What about in cases where you can't use POSIX functions for portability?

编辑：有关在32位和64位架构有效的解决方案是什么（很多code房源看起来他们会只在32位整数工作）

What about a solution that works on both 32 and 64 bit architectures (many of the code listings seem like they'd only work on 32 bit ints).

推荐答案

GCC已的：

 -- Built-in Function: int __builtin_clz (unsigned int x)
     Returns the number of leading 0-bits in X, starting at the most
     significant bit position.  If X is 0, the result is undefined.

 -- Built-in Function: int __builtin_clzl (unsigned long)
     Similar to `__builtin_clz', except the argument type is `unsigned
     long'.

 -- Built-in Function: int __builtin_clzll (unsigned long long)
     Similar to `__builtin_clz', except the argument type is `unsigned
     long long'.

我希望他们被翻译成一些合理有效为当前的平台，无论是那些花哨位变换算法中的一个，或单指令。

I'd expect them to be translated into something reasonably efficient for your current platform, whether it be one of those fancy bit-twiddling algorithms, or a single instruction.

这篇关于什么是找到在C中的整数集最高位（MSB）最快/最有效的方法是什么？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！