LinuxQuestions.org - constexpr and performance result

Quote:

Originally Posted by pan64 (Post 6489420)

would be nice to demonstrate it somehow. Or at least explain how did you measure it?

Il write simple benchmark for this, i not shure is correct:

Code:

#include <iostream>

#include <chrono>



constexpr auto fib_constexpr(unsigned int n) -> unsigned int {

        return (n <= 1) ? n : fib_constexpr(n - 1) + fib_constexpr(n - 2); 

}



auto fib_normal(unsigned int n) -> unsigned int {

        return (n <= 1) ? n : fib_normal(n - 1) + fib_normal(n - 2);

}



auto benchmark_fib_constexpr() -> void {



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

        auto end = std::chrono::system_clock::now();



        unsigned int count = 0;

        constexpr unsigned int n = 20;



        for(unsigned int i = 0; i < 10;) {

                fib_constexpr(n);

                ++count;

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

                if(std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count() >= 1000) {

                        std::cout<<"In second operation count is: "<<count<<std::endl;

                        count = 0;

                        ++i;

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

                }



        }        

}



auto benchmark_fib_normal() -> void {



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

        auto end = std::chrono::system_clock::now();



        unsigned int count = 0;

        unsigned int n = 20;



        for(unsigned int i = 0; i < 10;) {

                fib_normal(n);

                ++count;

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

                if(std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count() >= 1000) {

                        std::cout<<"In second operations count is: "<<count<<std::endl;

                        count = 0;

                        ++i;

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

                }

        }

}





auto main() -> int {

        std::cout<<"CONSTEXPR FIB BENCHMARK"<<std::endl;

        benchmark_fib_constexpr();

        std::cout<<"NORMAL FIB BENCHMARK"<<std::endl;

        benchmark_fib_normal();

        return (0);

}

important is compile with -O2 flag:

Code:

g++ -std=c++11 -O2 test.cpp

then in terminal i have result:

Code:

CONSTEXPR FIB BENCHMARK

In second operation count is: 49968765

In second operation count is: 49981539

In second operation count is: 50005027

In second operation count is: 49964753

In second operation count is: 49948262

In second operation count is: 49992233

In second operation count is: 49954760

In second operation count is: 49976978

In second operation count is: 49989060

In second operation count is: 49987518

NORMAL FIB BENCHMARK

In second operations count is: 131435

In second operations count is: 131457

In second operations count is: 131451

In second operations count is: 130226

In second operations count is: 130350

In second operations count is: 130034

In second operations count is: 129069

In second operations count is: 129363

In second operations count is: 120881

In second operations count is: 126952