a.c：
1 /************************************************************************* 2 * File: a.c 3 * Brief: 4 * Author: 5 * Mail: 6 * Created Time: Mon Dec 29 09:28:35 2014 7 ************************************************************************/ 8 9 #include<stdio.h> 10 11 int clz(unsigned int a) 12 { 13 return __builtin_clz(a); 14 } 15 16 int main() 17 { 18 unsigned int a=0; 19 a=0; 20 printf("0x%x ret=%d ", a, clz(a)); 21 22 a=1; 23 printf("0x%x ret=%d ", a, clz(a)); 24 25 a=2; 26 printf("0x%x ret=%d ", a, clz(a)); 27 28 a=4; 29 printf("0x%x ret=%d ", a, clz(a)); 30 31 a=8; 32 printf("0x%x ret=%d ", a, clz(a)); 33 34 a=16; 35 printf("0x%x ret=%d ", a, clz(a));; 36 37 a=0x0FFFFFFF; 38 printf("0x%x ret=%d ", a, clz(a)); 39 40 a=0x1FFFFFFF; 41 printf("0x%x ret=%d ", a, clz(a)); 42 43 a=0x2FFFFFFF; 44 printf("0x%x ret=%d ", a, clz(a)); 45 46 a=0x4FFFFFFF; 47 printf("0x%x ret=%d ", a, clz(a)); 48 49 a=0x8FFFFFFF; 50 printf("0x%x ret=%d ", a, clz(a)); 51 52 a=0xFFFFFFFF; 53 printf("0x%x ret=%d ", a, clz(a)); 54 55 return 0; 56 } 57
测试结果如下：
注意，a=0的时候，__builtin_cl返回的值和a=1的情况一样，都是是31位。
好奇怪，查到gcc官网上说(https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html)：
— 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.
也就是说，a=0的情况并没有规定在处理逻辑里面的。
下面我实现一个vc版本的：
int builtin_clz(unsigned int type) { int num = 0; type |=1; //防止type为0时，出现无限循环infinite loop，type为0时的计算结果为31。 while(!(type & 0x80000000)) //检测最高位是不是1。 { num +=1; type <<= 1; } return num; }
完。