为什么std :: sin()和std :: cos()比sin()和cos()慢？

Question

测试代码:

#include <cmath>
#include <cstdio>

const int N = 4096;
const float PI = 3.1415926535897932384626;

float cosine[N][N];
float sine[N][N];

int main() {
    printf("a\n");
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            cosine[i][j] = cos(i*j*2*PI/N);
            sine[i][j] = sin(-i*j*2*PI/N);
        }
    }
    printf("b\n");
}

这是时间:

$ g++ main.cc -o main
$ time ./main
a
b

real    0m1.406s
user    0m1.370s
sys     0m0.030s

添加后using namespace std;,时间是:

$ g++ main.cc -o main
$ time ./main
a
b

real    0m8.743s
user    0m8.680s
sys     0m0.030s

编译:

$ g++ --version
g++ (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2

部件:

Dump of assembler code for function sin@plt:                                    
0x0000000000400500 <+0>:     jmpq   *0x200b12(%rip)        # 0x601018 <_GLOBAL_OFFSET_TABLE_+48>
0x0000000000400506 <+6>:     pushq  $0x3                                     
0x000000000040050b <+11>:    jmpq   0x4004c0                                 
End of assembler dump.

Dump of assembler code for function std::sin(float):                            
0x0000000000400702 <+0>:     push   %rbp                                     
0x0000000000400703 <+1>:     mov    %rsp,%rbp                                
0x0000000000400706 <+4>:     sub    $0x10,%rsp                               
0x000000000040070a <+8>:     movss  %xmm0,-0x4(%rbp)                         
0x000000000040070f <+13>:    movss  -0x4(%rbp),%xmm0                         
0x0000000000400714 <+18>:    callq  0x400500 <sinf@plt>                      
0x0000000000400719 <+23>:    leaveq                                          
0x000000000040071a <+24>:    retq                                            
End of assembler dump.

Dump of assembler code for function sinf@plt:                                   
0x0000000000400500 <+0>:     jmpq   *0x200b12(%rip)        # 0x601018 <_GLOBAL_OFFSET_TABLE_+48>
0x0000000000400506 <+6>:     pushq  $0x3                                     
0x000000000040050b <+11>:    jmpq   0x4004c0                                 
End of assembler dump.

Answer 1

你正在使用不同的重载:

尝试

        double angle = i*j*2*PI/N;
        cosine[i][j] = cos(angle);
        sine[i][j] = sin(angle);

无论有没有,它都应该执行相同的操作 using namespace std;

Answer 2

我猜不同之处在于,对于float和double,std :: sin()存在重载,而sin()只需要加倍.在浮点数的std :: sin()里面,可能有一个转换为double,然后调用std :: sin()为double,然后将结果转换回float,使其变慢.