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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: OT: Re: Sieve of Erastosthenes optimized to the max
Date: Mon, 26 Aug 2024 12:08:11 -0700
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Vir Campestris <vir.campestris@invalid.invalid> writes:

> On 20/08/2024 16:24, Bonita Montero wrote:
>
>> Am 20.08.2024 um 17:21 schrieb Bonita Montero:
>>
>>> Am 19.08.2024 um 22:23 schrieb Vir Campestris:
>>>
>>>> On 16/08/2024 18:35, Bonita Montero wrote:
>>>> <snip>
>>>>
>>>>> But basically I don't think it is a good idea to skip numbers
>>>>> exept multiples of two.  [...] I think this extra computation
>>>>> is higher than the time for the saved memory loads.

[...]

> I'm leaning towards thinking you may have a point on the mod30
> code.  There are more operations on the innermost loop with mod30,
> although the loop goes around fewer times.  It also means that the
> store is forced to be byte wide - I find that my original odd-only
> code is significantly faster - about 30% - when the store is 64
> bit wide rather than byte.  [...]

One motivation for choosing a mod30 representation is being able to
compute more primes -- almost twice as many.  Any machine with 64GB
of memory should be able to compute primes up to 1.5 trillion.
Even if an odds-only representation runs faster for more limited
sizes (and I'm not yet convinced that it does), it doesn't matter
if it's faster, because it doesn't do the job.  I was computing all
32-bit primes 20 years ago, back when 32-bit machines were a thing.
The challenge for today's world is correspondingly higher.