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From: Student Project <student@invalid.invalid>
Newsgroups: comp.lang.c++
Subject: Re: constexpr is really very smart!
Date: Wed, 18 Dec 2024 04:45:20 +0000
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On 17/12/2024 20:20, Keith Thompson wrote:
> Student Project <student@invalid.invalid> writes:
>> The constexpr is really very smart because it can speed up algorithms
>> 1000 times according to Dave, Microsoft retired engineer. He has proved it
>> by creating this video:
>>
>> <https://youtu.be/8-VZoXn8f9U?si=iy1UimoWcaLG31Xi>
>>
>> On my computer it took 270 microseconds to calculate fib(35) like in his
>> example. It was almost instant at the blink of the eyes.
> 
> Can you post the relevant code?
> 
> [...]
> 

Code in the boxed area below: (The purpose is to demonstrate constexpr NOT the recursive algorithm. The video is very clear about this). 


It is the same code as in the video. G++ gives you the best result after 
changing one line to:

/*constexpr*/ int result_c = fibonacci_c(num);

G++ result is (multiple runs) - All timings in milliseconds:
D:\CmdLine\C_Cpp\Chrono05>program
Fibonacci_c 9227465
Time taken: 9
Fibonacci 9227465
Time taken: 289

D:\CmdLine\C_Cpp\Chrono05>program
Fibonacci_c 9227465
Time taken: 4
Fibonacci 9227465
Time taken: 276

D:\CmdLine\C_Cpp\Chrono05>program
Fibonacci_c 9227465
Time taken: 8
Fibonacci 9227465
Time taken: 284

D:\CmdLine\C_Cpp\Chrono05>program
Fibonacci_c 9227465
Time taken: 5
Fibonacci 9227465
Time taken: 276

clang++ and Visual studio are the slowest. I don't know why.



<+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++>
#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";

}

<+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++