求log2(n)的整数部分的快速算法

求log2(n)的整数部分的快速算法

inline int log2_int(register int x){
register int ans = 0;
while((x >>= 1)) ++ ans;
return ans;
}

const unsigned int tabel[32] = {0,0,1,1,2,2,2,2,3,3,3,3,3,3,3,3,3,3,3,3,3,3};

inline unsigned int log2_int_test(register unsigned int x){
register unsigned int ans = 0;
if(x >= 65536){ans += 16;x >>= 16;}
if(x >= 256  ){ans += 8 ;x >>= 8 ;}
if(x >= 16   ){ans += 4 ;x >>= 4 ;}
return ans + tabel[x];
}

unsigned int LOG2(unsigned int x){
unsigned int ret;
__asm__ __volatile__ ("bsrl %1, %%eax":"=a"(ret):"m"(x));
return ret;
}