理解__builtin_clz特性

 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\n", a, clz(a));
21 
22     a=1;
23     printf("0x%x ret=%d\n", a, clz(a));
24 
25     a=2;
26     printf("0x%x ret=%d\n", a, clz(a));
27 
28     a=4;
29     printf("0x%x ret=%d\n", a, clz(a));
30 
31     a=8;
32     printf("0x%x ret=%d\n", a, clz(a));
33 
34     a=16;
35     printf("0x%x ret=%d\n", a, clz(a));;
36 
37     a=0x0FFFFFFF;
38     printf("0x%x ret=%d\n", a, clz(a));
39 
40     a=0x1FFFFFFF;
41     printf("0x%x ret=%d\n", a, clz(a));
42 
43     a=0x2FFFFFFF;
44     printf("0x%x ret=%d\n", a, clz(a));
45 
46     a=0x4FFFFFFF;
47     printf("0x%x ret=%d\n", a, clz(a));
48 
49     a=0x8FFFFFFF;
50     printf("0x%x ret=%d\n", a, clz(a));
51 
52     a=0xFFFFFFFF;
53     printf("0x%x ret=%d\n", a, clz(a));
54 
55     return 0;
56 }
57          

— 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.

int builtin_clz(unsigned int type)
{
    int num = 0;
    while(!(type & 0x80000000)) //检测最高位是不是1.
    {
        num +=1;
        type <<= 1;
    }
    return num;
}