— Built-in Function: int __builtin_ffs (unsigned int x)
Returns one plus the index of the least significant 1-bit of x, or if x is zero, returns zero.
返回右起第一个‘1’的位置。
— 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.
返回左起第一个‘1’之前0的个数。
— Built-in Function: int __builtin_ctz (unsigned int x)
Returns the number of trailing 0-bits in x, starting at the least significant bit position. If x is 0, the result is undefined.
返回右起第一个‘1’之后的0的个数。
— Built-in Function: int __builtin_popcount (unsigned int x)
Returns the number of 1-bits in x.
返回‘1’的个数。
— Built-in Function: int __builtin_parity (unsigned int x)
Returns the parity of x, i.e. the number of 1-bits in x modulo 2.
返回‘1’的个数的奇偶性。
— Built-in Function: int __builtin_ffsl (unsigned long)
Similar to __builtin_ffs, except the argument type is unsigned long.
— Built-in Function: int __builtin_clzl (unsigned long)
Similar to __builtin_clz, except the argument type is unsigned long.
— Built-in Function: int __builtin_ctzl (unsigned long)
Similar to __builtin_ctz, except the argument type is unsigned long.
— Built-in Function: int __builtin_popcountl (unsigned long)
Similar to __builtin_popcount, except the argument type is unsigned long.
— Built-in Function: int __builtin_parityl (unsigned long)
Similar to __builtin_parity, except the argument type is unsigned long.
— Built-in Function: int __builtin_ffsll (unsigned long long)
Similar to __builtin_ffs, except the argument type is unsigned long long.
— Built-in Function: int __builtin_clzll (unsigned long long)
Similar to __builtin_clz, except the argument type is unsigned long long.
— Built-in Function: int __builtin_ctzll (unsigned long long)
Similar to __builtin_ctz, except the argument type is unsigned long long.
— Built-in Function: int __builtin_popcountll (unsigned long long)
Similar to __builtin_popcount, except the argument type is unsigned long long.
— Built-in Function: int __builtin_parityll (unsigned long long)
Similar to __builtin_parity, except the argument type is unsigned long long.
【实验程序】
1 #include <stdio.h>
2 #include <iostream>
3 #include <bitset>
4 #include <string>
5 #include <limits.h>
6
7 using namespace std;
8
9 uint32_t g_arr[12] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, UINT_MAX-1, UINT_MAX};
10
11 string g_str_func[] = {
12 "__builtin_ffs",
13 "__builtin_clz",
14 "__builtin_ctz",
15 "__builtin_popcount",
16 "__builtin_parity"
17 };
18
19 //typedef int (*fp_builtin_t)(unsigned int);
20
21 //fp_builtin_t g_func[] = {
22 //__builtin_ffs,
23 //__builtin_clz,
24 //__builtin_ctz,
25 //__builtin_popcount,
26 //__builtin_parity
27 //};
28
29 int main()
30 {
31 /* for (int k = 0; k < 5; k ++) {
32 printf("%s:\n", g_str_func[k].c_str());
33 for (int i = 0; i < 12; i ++) {
34 int t = g_arr[i];
35 bitset<32> b(t);
36 fp_builtin_t fp_func = g_func[k];
37 printf("%u(%s): %d\n", t, b.to_string().c_str(), fp_func(t));
38 }
39
40 puts("");
41 }
42 */
43
44 // ffs
45 printf("%s:\n", g_str_func[0].c_str());
46 for (int i = 0; i < 12; i ++) {
47 int t = g_arr[i];
48 bitset<32> b(t);
49 printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_ffs(t));
50 }
51 puts("");
52
53 // clz
54 printf("%s:\n", g_str_func[1].c_str());
55 for (int i = 0; i < 12; i ++) {
56 int t = g_arr[i];
57 bitset<32> b(t);
58 printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_clz(t));
59 }
60 puts("");
61
62 // ctz
63 printf("%s:\n", g_str_func[2].c_str());
64 for (int i = 0; i < 12; i ++) {
65 int t = g_arr[i];
66 bitset<32> b(t);
67 printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_ctz(t));
68 }
69 puts("");
70
71 // popcount
72 printf("%s:\n", g_str_func[3].c_str());
73 for (int i = 0; i < 12; i ++) {
74 int t = g_arr[i];
75 bitset<32> b(t);
76 printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_popcount(t));
77 }
78 puts("");
79
80 // parity
81 printf("%s:\n", g_str_func[4].c_str());
82 for (int i = 0; i < 12; i ++) {
83 int t = g_arr[i];
84 bitset<32> b(t);
85 printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_parity(t));
86 }
87 puts("");
88 return 0;
89 }
【测试结果】
1 __builtin_ffs: 2 0(00000000000000000000000000000000): 0 3 1(00000000000000000000000000000001): 1 4 2(00000000000000000000000000000010): 2 5 3(00000000000000000000000000000011): 1 6 4(00000000000000000000000000000100): 3 7 5(00000000000000000000000000000101): 1 8 6(00000000000000000000000000000110): 2 9 7(00000000000000000000000000000111): 1 10 8(00000000000000000000000000001000): 4 11 9(00000000000000000000000000001001): 1 12 4294967294(11111111111111111111111111111110): 2 13 4294967295(11111111111111111111111111111111): 1 14 15 __builtin_clz: 16 0(00000000000000000000000000000000): 31 17 1(00000000000000000000000000000001): 31 18 2(00000000000000000000000000000010): 30 19 3(00000000000000000000000000000011): 30 20 4(00000000000000000000000000000100): 29 21 5(00000000000000000000000000000101): 29 22 6(00000000000000000000000000000110): 29 23 7(00000000000000000000000000000111): 29 24 8(00000000000000000000000000001000): 28 25 9(00000000000000000000000000001001): 28 26 4294967294(11111111111111111111111111111110): 0 27 4294967295(11111111111111111111111111111111): 0 28 29 __builtin_ctz: 30 0(00000000000000000000000000000000): 0 31 1(00000000000000000000000000000001): 0 32 2(00000000000000000000000000000010): 1 33 3(00000000000000000000000000000011): 0 34 4(00000000000000000000000000000100): 2 35 5(00000000000000000000000000000101): 0 36 6(00000000000000000000000000000110): 1 37 7(00000000000000000000000000000111): 0 38 8(00000000000000000000000000001000): 3 39 9(00000000000000000000000000001001): 0 40 4294967294(11111111111111111111111111111110): 1 41 4294967295(11111111111111111111111111111111): 0 42 43 __builtin_popcount: 44 0(00000000000000000000000000000000): 0 45 1(00000000000000000000000000000001): 1 46 2(00000000000000000000000000000010): 1 47 3(00000000000000000000000000000011): 2 48 4(00000000000000000000000000000100): 1 49 5(00000000000000000000000000000101): 2 50 6(00000000000000000000000000000110): 2 51 7(00000000000000000000000000000111): 3 52 8(00000000000000000000000000001000): 1 53 9(00000000000000000000000000001001): 2 54 4294967294(11111111111111111111111111111110): 31 55 4294967295(11111111111111111111111111111111): 32 56 57 __builtin_parity: 58 0(00000000000000000000000000000000): 0 59 1(00000000000000000000000000000001): 1 60 2(00000000000000000000000000000010): 1 61 3(00000000000000000000000000000011): 0 62 4(00000000000000000000000000000100): 1 63 5(00000000000000000000000000000101): 0 64 6(00000000000000000000000000000110): 0 65 7(00000000000000000000000000000111): 1 66 8(00000000000000000000000000001000): 1 67 9(00000000000000000000000000001001): 0 68 4294967294(11111111111111111111111111111110): 1 69 4294967295(11111111111111111111111111111111): 0 70 71 72 Process returned 0 (0x0) execution time : 2.405 s 73 Press any key to continue.
这里存在一个为题，希望知道的朋友能够解释，就是为什么不能用函数指针指向这些内置函数。
答：为了性能，这些函数都是内联的。
比如很多平台都有特定的指令，所以这些“函数”都只要一条指令就完成了，做成函数得不偿失。
转自：https://www.cnblogs.com/nysanier/archive/2011/04/19/2020778.html
原文地址：https://www.cnblogs.com/zl1991/p/8192509.html