至少显著位的位置，它被设置 [英] Position of least significant bit that is set

问题描述

我要寻找一种有效的方法，以确定在一个整数设置最低显著位的位置，例如为0x0FF0这将是4。

I am looking for an efficient way to determine the position of the least significant bit that is set in an integer, e.g. for 0x0FF0 it would be 4.

一个简单的实现是这样的：

A trivial implementation is this:

unsigned GetLowestBitPos(unsigned value)
{
   assert(value != 0); // handled separately

   unsigned pos = 0;
   while (!(value & 1))
   {
      value >>= 1;
      ++pos;
   }
   return pos;
}

任何想法如何挤一些循环出来的吗？

Any ideas how to squeeze some cycles out of it?

（注意：这个问题是该享受这样的事情，没有人告诉我xyzoptimization是邪恶的人。）

(Note: this question is for people that enjoy such things, not for people to tell me xyzoptimization is evil.)

感谢大家的想法！我学到了一些其他的东西了。太酷了！

[edit] Thanks everyone for the ideas! I've learnt a few other things, too. Cool!

推荐答案

位操作黑客提供了一个极好的收集的，呃，有点摆弄黑客，具有性能/优化的讨论附后。我最喜欢你的问题的解决方案（从该站点）是«繁殖，并查找»：

Bit Twiddling Hacks offers an excellent collection of, er, bit twiddling hacks, with performance/optimisation discussion attached. My favourite solution for your problem (from that site) is «multiply and lookup»:

unsigned int v;  // find the number of trailing zeros in 32-bit v 
int r;           // result goes here
static const int MultiplyDeBruijnBitPosition[32] = 
{
  0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8, 
  31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
};
r = MultiplyDeBruijnBitPosition[((uint32_t)((v & -v) * 0x077CB531U)) >> 27];

有用的参考资料：

Helpful references:

• 使用德布鲁因序列在计算机的一个字指数1 - 解释一下，为什么上述code ++工程。


• 董事会重新presentation>位棋盘> BitScan - 这个问题的详细分析，尤其侧重于国际象棋编程

• "Using de Bruijn Sequences to Index a 1 in a Computer Word" - Explanation about why the above code works.
• "Board Representation > Bitboards > BitScan" - Detailed analysis of this problem, with a particular focus on chess programming

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