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Groups > comp.lang.c++ > #121254
| From | Tim Rentsch <tr.17687@z991.linuxsc.com> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: constexpr is really very smart! |
| Date | 2024-12-18 09:45 -0800 |
| Organization | A noiseless patient Spider |
| Message-ID | <867c7whol9.fsf@linuxsc.com> (permalink) |
| References | <vjndub$2glcu$1@paganini.bofh.team> <20241216112808.00003f74@yahoo.com> <86jzbyghdw.fsf@linuxsc.com> <20241218013342.0000518a@yahoo.com> |
Michael S <already5chosen@yahoo.com> writes:
> On Tue, 17 Dec 2024 12:54:19 -0800
> Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
>
>> Michael S <already5chosen@yahoo.com> writes:
>>
[...]
>>> #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;
>>> }
>>
>> Here is my second fastest fibonacci calculation code (for
>> relatively small inputs):
>>
>> typedef long unsigned long ULL;
>>
>> ULL
>> fibonacci( unsigned n ){
>> ULL b = n&1, a = b^1;
>>
>> if( n & 2 ) a += b, b += a;
>> if( n & 4 ) a += b, b += a, a += b, b += a;
>> if( n & 8 ){
>> ULL na = 13*a+21*b, nb = 21*a+34*b;
>> a = na, b = nb;
>> }
>>
>> n >>= 4;
>> while( n-- ){
>> ULL na = 610*a + 987*b, nb = 987*a + 1597*b;
>> a = na, b = nb;
>> }
>>
>> return b;
>> }
>
> From BigO perspective this code looks like it's still O(n) so no
> better than simple loop.
It is O(n) but it has a smaller constant.
> According to my measurement gear, in range 0 to 92 there are few
> points where it is faster than simple loop, but in majority of
> cases it is slower.
I'm at a loss to understand how this could happen. In my own
measurements, the code shown above runs faster than a simple loop in
all cases above n > 12, more than twice as fast when n > 17, more
than three times as fast when n > 42, and going up from there. What
might account for these radically different results?
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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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