c - 计算任何数字的 log(base 2) 的最佳方法是什么？

Question

我需要在 Linux 内核编程中找到任意数字的 log(base 2)。此操作是否有任何内置功能？如果不是怎么计算？

Answer 1

如果整数结果对您来说足够了，您可以将多次除以 2。类似

int logFunc(unsigned int x) 
{ 
   int log = -1; 
   while(x) { 
    log++; 
    x >>= 1; 
   } 
   return log; 
} 

如果你需要 fp 操作，你应该阅读这个:

[...]if the question was "can I just use FP in the kernel" then the answer is still a resounding NO, since other architectures may not support it AT ALL.

  Linus

Link

此外，您还可以查看:Use of floating point in the Linux kernel

编辑:如果您需要更快的版本，您可以阅读 Bit Twiddling Hacks - By Sean Eron Anderson

uint32_t v; // find the log base 2 of 32-bit v
int r;      // result goes here

static const int MultiplyDeBruijnBitPosition[32] = 
{
  0, 9, 1, 10, 13, 21, 2, 29, 11, 14, 16, 18, 22, 25, 3, 30,
  8, 12, 20, 28, 15, 17, 24, 7, 19, 27, 23, 6, 26, 5, 4, 31
};

v |= v >> 1; // first round down to one less than a power of 2 
v |= v >> 2;
v |= v >> 4;
v |= v >> 8;
v |= v >> 16;

r = MultiplyDeBruijnBitPosition[(uint32_t)(v * 0x07C4ACDDU) >> 27];