Re: constexpr is really very smart!

On 16/12/2024 10:28, Michael S wrote:

On Sun, 15 Dec 2024 20:20:42 +0000
Student Project <student@invalid.invalid> wrote:
 

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.
>

D:\CmdLine\C_Cpp\Chrono02>program
Fibonacci_c: 9227465
Time Taken: 270
D:\CmdLine\C_Cpp\Chrono02>program
Fibonacci_c: 9227465
Time Taken: 257
D:\CmdLine\C_Cpp\Chrono02>program
Fibonacci_c: 9227465
Time Taken: 171
D:\CmdLine\C_Cpp\Chrono02>program
Fibonacci_c: 9227465
Time Taken: 176

>
Amazing.
>

 I didn't see the video (I never see that type of videos), but 270
microseconds sound astonishingly slow for fib(35).
 #include <stdio.h>
#include <stdlib.h>
#include <intrin.h>
 static long long fib(long n)
{
   if (fib <= 0)
     return 0;
   long long f0 = 0, f1 = 1;
   for (long i = 1; i < n; ++i) {
     long long f2 = f0 + f1;
     f0 = f1;
     f1 = f2;
   }
   return f1;
}
 

There are a dozen different ways to calculate Fibonacci numbers, ranging from very fast to very slow - but demonstrating different things.  Even your program is going to be very slow compared to just using φⁿ / √5.
My guess is that the OP is referring to some kind of naïve recursive Fibonacci implementation.  If the function is declared "constexpr" and the values used are compile-time constants, then maybe the use of "constexpr" leads to more of it being pre-calculated at compile time.
The OP is right that "constexpr" can be very "smart" - but I'm not sure this is the best calculation for demonstrating that.  However, I have not watched the video either, and maybe it's well presented.