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 21:47:57 +0100 [thread overview]
Message-ID: <f7a9d795-9f96-b8b1-21b2-90ce31c2c724@gmail.com> (raw)
In-Reply-To: <ab35dac8a309f8a0f358cd9e3df382d760d5b125@opteya.com>
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Hi Yann,
On 12/30/22 21:18, Yann Droneaud wrote:
> 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;
>
> unsigned long max = upper_bound - 1;
> unsigned long mask = ULONG_MAX >> __builtin_clzl(max);
>
> do {
> r = shadow_random();
> r &= mask;
> } while (r > max);
>
> return r;
>
Based on some of your suggestions, I updated it to be the following:
unsigned long
shadow_random_uniform(unsigned long upper_bound)
{
unsigned long r, max, mask;
max = upper_bound - 1;
mask = bit_ceilul(upper_bound) - 1;
do {
r = shadow_random();
r &= mask; // optimization
} while (r > max);
return r;
}
See how upper_bound == 0 acts as if upper_bound had a value one more than the
maximum representable value in the type, which is a nice property.
Cheers,
Alex
--
<http://www.alejandro-colomar.es/>
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next prev parent reply other threads:[~2022-12-30 20:48 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
2022-12-30 22:33 ` Alejandro Colomar
2023-01-06 20:08 ` Alejandro Colomar
2022-12-30 20:47 ` Alejandro Colomar [this message]
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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