if/else优化测试结果不明确

Question

我正在对代码中的三个场景进行基准测试，在这些场景中，我希望优化掉一个if语句(该值只在运行时设置一次，然后检查数百万次)。我评估过的三种方法如下

  1. A template bool binded at compile time
  2. A static bool initialized on first call
  3. A bool member variable which gets initialized at construction

结果有点不直观，即模板版本花费了相同的时间，但静态成员变量方法更快。有人能解释一下原因吗？

结果(在多次运行时保持一致)

elapsed time: 0.230933 4501s
elapsed time: 0.212918 4501s
elapsed time: 0.230512 4501s

示例代码如下

#ifndef TEST_H_
#define TEST_H_

#include <math.h>
#include <iostream>

template<bool Condition>
class Test {
public:
    Test() :
        signal_(0.0), condition_(false), max_truncation_(12000.00) {
    }
    ~Test() {
    }

    void CalculateTestingTemplate(double& i);

    void CalculateTestingStatic(double& i);
    void CalculateTestingMemberVariable(double& i);

private:
    double signal_;
    double condition_;

    double max_truncation_;
};

template<bool Condition>
void Test<Condition>::CalculateTestingTemplate(double& i) {
    i++;

    if (Condition) {
        if (fabs(i) > max_truncation_) {
            i = 0;
        }
    }

    if (Condition) {
        if (fabs(i) > max_truncation_) {
            i = 0;
        }
    }

    if (fabs(i) > max_truncation_) {
        i = 0;
    }
};

template<bool Condition>
void Test<Condition>::CalculateTestingStatic(double& i) {
    i++;
    static bool condition = condition_;
    if (condition) {
        if (fabs(i) > max_truncation_) {
            i = 0;
        }
    }

    if (condition) {
        if (fabs(i) > max_truncation_) {
            i = 0;
        }
    }

    if (fabs(i) > max_truncation_) {
        i = 0;
    }
};


template<bool Condition>
void Test<Condition>::CalculateTestingMemberVariable(double& i) {
    i++;
    if (condition_) {
        if (fabs(i) > max_truncation_) {
            i = 0;
        }
    }

    if (condition_) {
        if (fabs(i) > max_truncation_) {
            i = 0;
        }
    }
    if (fabs(i) > max_truncation_) {
        i = 0;
    }
};

#endif // TEST_H_

和循环计时的主要函数(-O3)

#include <ctime>
#include <chrono>

#include "test.h"

using namespace std;

int main(int argc, char const *argv[]) {

    Test<false>* object = new Test<false>();
    int iteration = 90000000;

    std::chrono::time_point<std::chrono::system_clock> start, end;
    start = std::chrono::system_clock::now();

    int i = 0;
    double j = 0;
    for(; i<iteration;i++)
    {
        object->CalculateTestingTemplate(j);
    }
    end = std::chrono::system_clock::now();
    std::chrono::duration<double> elapsed_seconds = end - start;
    std::cout << "elapsed time: " << elapsed_seconds.count() << " " << j << "s\n";

    start = std::chrono::system_clock::now();

    i = 0;
    j = 0;
    for(; i<iteration;i++)
    {
        object->CalculateTestingStatic(j);
    }
    end = std::chrono::system_clock::now();
    elapsed_seconds = end - start;
    std::cout << "elapsed time: " << elapsed_seconds.count() << " " << j << "s\n";


    start = std::chrono::system_clock::now();

    i = 0;
    j = 0;
    for(; i<iteration;i++)
    {
        object->CalculateTestingMemberVariable(j);
    }

    end = std::chrono::system_clock::now();
    elapsed_seconds = end - start;
    std::cout << "elapsed time: " << elapsed_seconds.count() << " " << j << "s\n";


    return 0;
}

Answer 1

您的模板测试可能比预期的慢，因为它首先运行，因此遇到缓存未命中，而其他测试不会遇到缓存未命中，因为模板测试已经将数据带入缓存。