基准测试结果奇怪

Willi Mentzel 发表于 Dev

威利·曼策尔

我编写了以下基准：

#include <iostream> // cout
#include <math.h>   // pow
#include <chrono>   // high_resolution_clock    

using namespace std;
using namespace std::chrono;

int64_t calculate(int);

int main()
{
    high_resolution_clock::time_point t1, t2;

    // Test 1
    t1 = high_resolution_clock::now();
    calculate(200);
    t2 = high_resolution_clock::now();

    cout << "RUNTIME = " <<  duration_cast<nanoseconds>(t2 - t1).count() << " nano seconds" << endl;

    // Test 2   
    t1 = high_resolution_clock::now();
    calculate(200000);
    t2 = high_resolution_clock::now();

    cout << "RUNTIME = " <<  duration_cast<nanoseconds>(t2 - t1).count() << " nano seconds" << endl;
}

int64_t calculate(const int max_exponent)
{
    int64_t num = 0;

    for(int i = 0; i < max_exponent; i++)
    {
        num += pow(2, i);
    }

    return num;
}

在Odroid XU3上运行此基准测试时，将产生以下输出（运行8次）：

RUNTIME TEST 1 = 1250 nano seconds
RUNTIME TEST 2 = 1041 nano seconds

RUNTIME TEST 1 = 1292 nano seconds
RUNTIME TEST 2 = 1042 nano seconds

RUNTIME TEST 1 = 1250 nano seconds
RUNTIME TEST 2 = 1083 nano seconds

RUNTIME TEST 1 = 1292 nano seconds
RUNTIME TEST 2 = 1083 nano seconds

RUNTIME TEST 1 = 1209 nano seconds
RUNTIME TEST 2 = 1084 nano seconds

RUNTIME TEST 1 = 1166 nano seconds
RUNTIME TEST 2 = 1083 nano seconds

RUNTIME TEST 1 = 1292 nano seconds
RUNTIME TEST 2 = 1042 nano seconds

RUNTIME TEST 1 = 1166 nano seconds
RUNTIME TEST 2 = 1250 nano seconds

RUNTIME TEST 1 = 1250 nano seconds
RUNTIME TEST 2 = 1250 nano seconds

第二个指数是第一个指数的1000倍。为什么第二通电话有时会更快结束？

我将GCC（4.8）用作带有-Ofast标志的编译器。

更新：我可以在我的i7 4770k上重现类似的行为。

杰里·科芬

简短的答案是“消除死代码”。编译器会发现您永远不会使用调用该函数的结果（并且该函数没有副作用），因此它仅消除了调用该函数。

打印出函数的结果，情况会有所变化。例如：

Ignore: -9223372036854775808    RUNTIME = 0 nano seconds
Ignore: -9223372036854775808    RUNTIME = 23001300 nano seconds

修改后的代码，以防万一：

#include <iostream> // cout
#include <math.h>   // pow
#include <chrono>   // high_resolution_clock    

using namespace std;
using namespace std::chrono;

int64_t calculate(int);

int main() {
    high_resolution_clock::time_point t1, t2;

    // Test 1
    t1 = high_resolution_clock::now();
    auto a = calculate(200);
    t2 = high_resolution_clock::now();
    std::cout << "Ignore: " << a << "\t";

    cout << "RUNTIME = " << duration_cast<nanoseconds>(t2 - t1).count() << " nano seconds" << endl;

    // Test 2   
    t1 = high_resolution_clock::now();
    auto b = calculate(200000);
    t2 = high_resolution_clock::now();
    std::cout << "Ignore: " << b << "\t";

    cout << "RUNTIME = " << duration_cast<nanoseconds>(t2 - t1).count() << " nano seconds" << endl;
}

int64_t calculate(const int max_exponent) {
    int64_t num = 0;

    for (int i = 0; i < max_exponent; i++) {
        num += pow(2, i);
    }

    return num;
}