Question

编辑：我知道分支预测失败，这是导致我提出这个问题的主要原因。如果有更多的分支机构，我预计会有更多失败的预测。除了第二部分中的额外分支外，我的代码的第一部分和第二部分完全相同。此额外分支将始终评估为true，但仍需要评估i％2。为什么它更快？编译器优化解决了这个问题，但是出现了一个新的问题。为什么在关闭优化时会发生这种情况？

原帖：我试图了解if和else语句如何影响我在C ++中的代码性能。这些是我使用的参数和代码：编译器：g++ (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609 编译：{{1}}

g++ -Wall ifs_comparison.cpp -o ifs_comparison

输出结果为：

int main(){
    clock_t t; 
    int N = 100000000;
    int even_odd[2];

    // Using if statements and being stupid with the else statement.
    // This uses more calculations than the ones that should be needed.
    even_odd[0] = 0;
    even_odd[1] = 0;
    t = clock();
    for(int i = 0; i < N; i++){
        if(i%2==0)
            even_odd[0]++;
        else
        {
            if(i%2==1)
                even_odd[1]++;
        }
    }
    t = clock() - t;
    std::cout << "Using 'if' statements and being stupid with the 'else' statement.\n";
    std::cout << "This took " << ((float)t)/CLOCKS_PER_SEC << "seconds\n";
    std::cout << "Repeat the task but with a simpler 'else' statement\n";




    // Using if statements but simplifying the else statement
    // Reset the variables
    even_odd[0] = 0;
    even_odd[1] = 0;    
    t = clock();
    for(int j = 0; j < N; j++)
    {
        if(j%2==0)
            even_odd[0]++;
        else
            even_odd[1]++;
    }   
    t = clock() - t;
    std::cout << "This took " << ((float)t)/CLOCKS_PER_SEC << "seconds\n";
    std::cout << "Repeat the task but without 'if' statements\n";



    // Not using 'if' statements
    // Reset the variables
    even_odd[0] = 0;
    even_odd[1] = 0;
    t = clock();    
    for(int k = 0; k < N; k++)
            even_odd[k%2]++;
    t = clock()-t;
    std::cout << "This took " << ((float)t)/CLOCKS_PER_SEC << "seconds\n";

    return 0;
}

正如所料，没有Using 'if' statements and being stupid with the 'else' statement. This took 0.262272seconds Repeat the task but with a simpler 'else' statement This took 0.294445seconds Repeat the task but without 'if' statements This took 0.238543seconds语句的部分运行得更快。因此，如果我通过删除评估if简化了else语句，我想我还能看到它的速度有多快。令我惊讶的是，它实际上更慢！发生了什么事？有人能指出我对这些i%2和if语句如何工作的一个很好的参考？

C ++＆＃39;如果＆＃39;分支：更多分支运行得更快？

0 个答案: