Re: constexpr is really very smart!

On Wed, 18 Dec 2024 04:45:20 +0000
Student Project <student@invalid.invalid> wrote:

 
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#include <iostream>
#include <chrono>
 
int fibonacci(int n)
{
     if (n <= 1)
         return n;
     return fibonacci(n - 2) + fibonacci(n - 1);
}
 
constexpr int fibonacci_c(int n)
{
     if (n <= 1)
         return n;
     return fibonacci_c(n - 2) + fibonacci_c(n - 1);
}
 
int main(void)
{
     // using namespace std::literals::chrono_literals;
     auto start = std::chrono::high_resolution_clock::now();
     constexpr int num = 35;
     /*constexpr*/ int result_c = fibonacci_c(num);
 
     std::cout << "Fibonacci_c " << result_c << "\n";
     std::cout << "Time taken: " <<
std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::high_resolution_clock::now()
- start).count() << "\n";
 
     start = std::chrono::high_resolution_clock::now();
     int result = fibonacci(num);
     std::cout << "Fibonacci " << result << "\n";
     std::cout << "Time taken: " <<
std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::high_resolution_clock::now()
- start).count() << "\n";
 
}
 
<+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 

It is likely that at least part of the time you are measuring cout print
time instead of calculations time you are interested in.
Also, if you are using gcc on Windows then there is a significant chance
that implementation of high_resolution_clock is broken. steady_clock is
more reliable.

Try following:
 volatile dummy = 0;
 auto t00 = std::chrono::steady_clock::now();// first call can be slow
 dummy = 1;
 auto t0 = std::chrono::steady_clock::now();
 dummy = 2;
 auto result_c = fibonacci_c(num);
 dummy = 3;
 auto t1 = std::chrono::steady_clock::now();
 dummy = 4;
 auto result = fibonacci(num);
 dummy = 5;
 auto t2 = std::chrono::steady_clock::now();
 dummy = 3;

 etc...