From: Alejandro Colomar <alx.manpages@gmail.com>
To: Yann Droneaud <ydroneaud@opteya.com>,
Joseph Myers <joseph@codesourcery.com>
Cc: gcc@gcc.gnu.org, GNU C Library <libc-alpha@sourceware.org>
Subject: Re: stdc_bit_ceil(3) and wrapping
Date: Fri, 30 Dec 2022 21:33:42 +0100 [thread overview]
Message-ID: <bf20ade4-6823-d5c0-86be-8ef58e73d16d@gmail.com> (raw)
In-Reply-To: <ab35dac8a309f8a0f358cd9e3df382d760d5b125@opteya.com>
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Hi Yann,
On 12/30/22 21:18, Yann Droneaud wrote:
> Hi,
>
> 30 décembre 2022 à 20:55 "Alejandro Colomar via Libc-alpha" <libc-alpha@sourceware.org> a écrit:
>>
>> I'm implementing a small part of <stdbit.h> equivalent code for shadow. I need
>> stdc_bit_ceilul() for a random number generator limited to a range (you've seen
>> some of this in the glibc mailing list.
>>
>> $ grepc -tfd shadow_random_uniform
>> ./libmisc/random.c:76:
>> unsigned long
>> shadow_random_uniform(unsigned long upper_bound)
>> {
>> unsigned long r;
>>
>> do {
>> r = shadow_random();
>> r &= bit_ceil_wrapul(upper_bound) - 1; // optimization
>> } while (r > upper_bound - 1);
>>
>> return r;
>> }
>>
>
> What's wrong with the following ?
>
> if (upper_bound < 2)
> return 0;
If upper_bound is 1, the only valid value is 0, but if it is 0, I prefer it to
behave as if there was no bound at all, because then it allows a function
shadow_random_range(min, max) to be implemented as:
/*
* Return a uniformly-distributed random number in the inclusive range:
* [min, max]
*/
unsigned long
shadow_random_range(unsigned long min, unsigned long max)
{
return shadow_random_uniform(max - min + 1) + min;
}
That function has no problems when max is ULONG_MAX and min is 0, which is a
nice feature.
BTW, this is something that might be interesting for both rand(3) and
arc4random(3) in libc, since it's something that is error-prone to roll your own
wrapper around the *_uniform() function (for example, shadow had it biased, and
I'm fixing it).
If you want, I could prepare a patch for glibc.
>
> unsigned long max = upper_bound - 1;
> unsigned long mask = ULONG_MAX >> __builtin_clzl(max);
I hate coding these magic operations out of a function, when I can give it a
meaningful name. That reads to me as a magic trick that many maintainers that
read it after me will blame me for having to parse it.
Moreover, it requires you to have the special case for 0 at the top, which I
don't want.
>
> do {
> r = shadow_random();
> r &= mask;
Moving the calculation of the mask out of the loop is something I had in mind, yep.
I also considered reusing the remaining bits if possible, but I prefer to keep
the code simple, even if it has a few more calls to arc4random(3) underneath.
> } while (r > max);
Yeah, this max is more friendly than my magic -1. Thanks! :)
>
> return r;
>
Cheers,
Alex
--
<http://www.alejandro-colomar.es/>
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next prev parent reply other threads:[~2022-12-30 20:33 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-30 19:53 Alejandro Colomar
2022-12-30 20:18 ` Yann Droneaud
2022-12-30 20:33 ` Alejandro Colomar [this message]
2022-12-30 22:33 ` Alejandro Colomar
2023-01-06 20:08 ` Alejandro Colomar
2022-12-30 20:47 ` Alejandro Colomar
2022-12-30 20:38 ` Joseph Myers
2022-12-30 20:46 ` Alejandro Colomar
2022-12-30 20:56 ` Joseph Myers
2022-12-30 21:01 ` Alejandro Colomar
2022-12-30 21:06 ` Alejandro Colomar
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