Re: constexpr is really very smart!

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.