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Groups > comp.lang.c++ > #121216
| From | Michael S <already5chosen@yahoo.com> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: constexpr is really very smart! |
| Date | 2024-12-16 13:23 +0200 |
| Organization | A noiseless patient Spider |
| Message-ID | <20241216132315.00007859@yahoo.com> (permalink) |
| References | <vjndub$2glcu$1@paganini.bofh.team> <20241216112808.00003f74@yahoo.com> <vjouq6$12khu$2@dont-email.me> |
On Mon, 16 Dec 2024 11:18:46 +0100
David Brown <david.brown@hesbynett.no> wrote:
> 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 news reader is not sure about equation above.
You probably meant ceil(pow((1+sqrt(5))/2, n)/sqrt(5))
It works well with double precision math up to n=75. After that it
would need high precision fp library, so probably [much] slower than
simple loop above at least up to n=92 where we run out of range of int64
result.
> My guess is that the OP is referring to some kind of naïve recursive
> Fibonacci implementation.
Something like that ?
static long long slow_fib(long n)
{
if (n < 1)
return 0;
if (n == 1)
return 1;
return slow_fib(n-2)+slow_fib(n-1);
}
Yes, 270 usec could be considered fast relatively to that. slow_fib(35)
takes 20 msec on my old PC.
> 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.
>
But then, how long would it take to compile?
I'd guess for n=35 it's not too bad, but what about n=92?
Slow algorithm is a slow algorithm. Doing it in compile time can only
exaggerate the slowness.
> 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.
>
If it is video and it is about programming then it can not be good.
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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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