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Re: constexpr is really very smart!

From Michael S <already5chosen@yahoo.com>
Newsgroups comp.lang.c++
Subject Re: constexpr is really very smart!
Date 2024-12-16 11:28 +0200
Organization A noiseless patient Spider
Message-ID <20241216112808.00003f74@yahoo.com> (permalink)
References <vjndub$2glcu$1@paganini.bofh.team>

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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;
}

int main(int argz, char** argv)
{
  if (argz < 2) {
    printf("Go away!\n");
    return 1;
  }

  char* endp;
  long n = strtol(argv[1], &endp, 0);
  if (endp == argv[1]) {
    printf("%s is not a number. Go away!\n", argv[1]);
    return 1;
  }

  unsigned long long t0 = _rdtsc();
  long long f = fib(n);
  unsigned long long t1 = _rdtsc();
  printf("fib(%ld)=%lld. %lld clock cycles.\n", n, f, t1 - t0);
  return 0;
}

$ gcc -s -O2 -Wall fib.c -o fib
$ ./fib 35
fib(35)=9227465. 212 clock cycles.

212 cycles on this 11 y.o. CPU means 62 nanoseconds. Even that time, I
would guess, is mostly a measurement overhead.
$ ./fib 92
fib(92)=7540113804746346429. 281 clock cycles.

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Thread

constexpr is really very smart! Student Project <student@invalid.invalid> - 2024-12-15 20:20 +0000
  Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-16 11:28 +0200
    Re: constexpr is really very smart! David Brown <david.brown@hesbynett.no> - 2024-12-16 11:18 +0100
      Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-16 13:23 +0200
        Re: constexpr is really very smart! David Brown <david.brown@hesbynett.no> - 2024-12-16 13:43 +0100
          Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-17 11:08 +0200
            Re: constexpr is really very smart! David Brown <david.brown@hesbynett.no> - 2024-12-17 11:10 +0100
        Re: constexpr is really very smart! Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-18 03:26 -0800
          Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-18 14:39 +0200
            Re: constexpr is really very smart! Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-18 07:42 -0800
    Re: constexpr is really very smart! Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-17 12:54 -0800
      Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-18 01:33 +0200
        Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-18 13:51 +0200
          Re: constexpr is really very smart! Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-19 00:40 -0800
            Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-20 02:17 +0200
              Re: constexpr is really very smart! Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-20 14:59 -0800
                Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-22 00:07 +0200
                Re: constexpr is really very smart! Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-22 10:25 -0800
                Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-23 17:47 +0200
                Re: constexpr is really very smart! Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-24 05:21 -0800
                Bignum multiplication in Python vs GMP (was: constexpr is really very smart!) Michael S <already5chosen@yahoo.com> - 2024-12-25 11:05 +0200
                Re: Bignum multiplication in Python vs GMP (was: constexpr is really very smart!) Michael S <already5chosen@yahoo.com> - 2024-12-25 13:33 +0200
                Re: Bignum multiplication in Python vs GMP (was: constexpr is really very smart!) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-26 18:54 -0800
                Re: Bignum multiplication in Python vs GMP (was: constexpr is really very smart!) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-26 18:42 -0800
          Re: constexpr is really very smart! Student Project <student@invalid.invalid> - 2024-12-19 19:14 +0000
        Re: constexpr is really very smart! Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-18 09:45 -0800
          Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-18 22:00 +0200
            Re: constexpr is really very smart! Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-19 00:57 -0800
              Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-19 23:45 +0200
                Re: constexpr is really very smart! Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-20 14:31 -0800
                Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-22 00:14 +0200
                Re: constexpr is really very smart! James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-12-21 17:55 -0500
                Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-22 02:22 +0200
                Re: constexpr is really very smart! Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-22 10:33 -0800
                Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-23 00:05 +0200
                Re: constexpr is really very smart! Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-12-22 17:17 -0800
  Re: constexpr is really very smart! Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-12-17 12:20 -0800
    Re: constexpr is really very smart! Student Project <student@invalid.invalid> - 2024-12-18 04:45 +0000
      Re: constexpr is really very smart! Michael S <already5chosen@yahoo.com> - 2024-12-18 14:17 +0200

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