glibc的几个有用的处理二进制位的内置函数

作者：F.N.  来源：博客园  发布时间：2011-04-19 13:06  阅读：170 次  原文链接   [收藏]  — 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】#include #include #include #include #include  using namespace std; uint32_t g_arr[12] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, UINT_MAX-1, UINT_MAX}; string g_str_func[] = {    "__builtin_ffs",    "__builtin_clz",    "__builtin_ctz",    "__builtin_popcount",    "__builtin_parity"}; //typedef int (*fp_builtin_t)(unsigned int); //fp_builtin_t g_func[] = {    //__builtin_ffs,    //__builtin_clz,    //__builtin_ctz,    //__builtin_popcount,    //__builtin_parity//}; int main(){/*    for (int k = 0; k < 5; k ++) {        printf("%s:\n", g_str_func[k].c_str());        for (int i = 0; i < 12; i ++) {            int t = g_arr[i];            bitset<32> b(t);            fp_builtin_t fp_func = g_func[k];            printf("%u(%s): %d\n", t, b.to_string().c_str(), fp_func(t));        }         puts("");    }*/         // ffs        printf("%s:\n", g_str_func[0].c_str());        for (int i = 0; i < 12; i ++) {            int t = g_arr[i];            bitset<32> b(t);            printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_ffs(t));        }        puts("");         // clz        printf("%s:\n", g_str_func[1].c_str());        for (int i = 0; i < 12; i ++) {            int t = g_arr[i];            bitset<32> b(t);            printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_clz(t));        }        puts("");         // ctz        printf("%s:\n", g_str_func[2].c_str());        for (int i = 0; i < 12; i ++) {            int t = g_arr[i];            bitset<32> b(t);            printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_ctz(t));        }        puts("");         // popcount        printf("%s:\n", g_str_func[3].c_str());        for (int i = 0; i < 12; i ++) {            int t = g_arr[i];            bitset<32> b(t);            printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_popcount(t));        }        puts("");         // parity        printf("%s:\n", g_str_func[4].c_str());        for (int i = 0; i < 12; i ++) {            int t = g_arr[i];            bitset<32> b(t);            printf("%u(%s): %d\n", t, b.to_string().c_str(), __builtin_parity(t));        }        puts("");    return 0;}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。 【测试结果】view source print?__builtin_ffs:0(00000000000000000000000000000000): 01(00000000000000000000000000000001): 12(00000000000000000000000000000010): 23(00000000000000000000000000000011): 14(00000000000000000000000000000100): 35(00000000000000000000000000000101): 16(00000000000000000000000000000110): 27(00000000000000000000000000000111): 18(00000000000000000000000000001000): 49(00000000000000000000000000001001): 14294967294(11111111111111111111111111111110): 24294967295(11111111111111111111111111111111): 1 __builtin_clz:0(00000000000000000000000000000000): 311(00000000000000000000000000000001): 312(00000000000000000000000000000010): 303(00000000000000000000000000000011): 304(00000000000000000000000000000100): 295(00000000000000000000000000000101): 296(00000000000000000000000000000110): 297(00000000000000000000000000000111): 298(00000000000000000000000000001000): 289(00000000000000000000000000001001): 284294967294(11111111111111111111111111111110): 04294967295(11111111111111111111111111111111): 0 __builtin_ctz:0(00000000000000000000000000000000): 01(00000000000000000000000000000001): 02(00000000000000000000000000000010): 13(00000000000000000000000000000011): 04(00000000000000000000000000000100): 25(00000000000000000000000000000101): 06(00000000000000000000000000000110): 17(00000000000000000000000000000111): 08(00000000000000000000000000001000): 39(00000000000000000000000000001001): 04294967294(11111111111111111111111111111110): 14294967295(11111111111111111111111111111111): 0 __builtin_popcount:0(00000000000000000000000000000000): 01(00000000000000000000000000000001): 12(00000000000000000000000000000010): 13(00000000000000000000000000000011): 24(00000000000000000000000000000100): 15(00000000000000000000000000000101): 26(00000000000000000000000000000110): 27(00000000000000000000000000000111): 38(00000000000000000000000000001000): 19(00000000000000000000000000001001): 24294967294(11111111111111111111111111111110): 314294967295(11111111111111111111111111111111): 32 __builtin_parity:0(00000000000000000000000000000000): 01(00000000000000000000000000000001): 12(00000000000000000000000000000010): 13(00000000000000000000000000000011): 04(00000000000000000000000000000100): 15(00000000000000000000000000000101): 06(00000000000000000000000000000110): 07(00000000000000000000000000000111): 18(00000000000000000000000000001000): 19(00000000000000000000000000001001): 04294967294(11111111111111111111111111111110): 14294967295(11111111111111111111111111111111): 0  Process returned 0 (0x0)   execution time : 2.405 sPress any key to continue.