From: Eli Zaretskii <eliz@gnu.org>
To: Andy Moreton <andrewjmoreton@gmail.com>
Cc: 32605@debbugs.gnu.org
Subject: bug#32605: [w64] (random) never returns negative
Date: Fri, 13 Aug 2021 09:29:41 +0300 [thread overview]
Message-ID: <83czqhdfhm.fsf@gnu.org> (raw)
In-Reply-To: <86bl62s8qm.fsf@gmail.com> (message from Andy Moreton on Thu, 12 Aug 2021 21:34:09 +0100)
> From: Andy Moreton <andrewjmoreton@gmail.com>
> Date: Thu, 12 Aug 2021 21:34:09 +0100
>
> > int val = ((rand_as183 () << 15) | rand_as183 ());
> > #ifdef __x86_64__
> > return 2 * val - 0x3FFFFFFF;
> > #else
> > return val;
> > #endif
> >
> > Andy, can you test this, please?
>
> That does not produce any negative random numbers within a reasonable
> number of attempts (a few dozen calls).
Thanks for testing.
> Instead, calling rand_as183 again (as below) does produce positive and
> negative random numbers on 32bit and 64bit builds with a similar number
> of attempts:
>
> return ((rand_as183 () << 30) | (rand_as183 () << 15) | rand_as183 ());
>
> While this may be less efficient, it at least meets the contract of
> providing 31 random bits.
What about the variant below, does it produce better results?
int val = ((rand_as183 () << 15) | rand_as183 ());
#ifdef __x86_64__
return 2 * val - 0x7FFFFFFF;
#else
return val;
#endif
next prev parent reply other threads:[~2021-08-13 6:29 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-01 17:18 bug#32605: 26.1; (random) never returns negative Francis Wright
2018-09-01 17:34 ` Stephen Berman
2018-09-04 22:27 ` Noam Postavsky
2018-09-05 13:20 ` Andy Moreton
2021-08-12 13:17 ` bug#32605: [w64] " Lars Ingebrigtsen
2021-08-12 13:42 ` Eli Zaretskii
2021-08-12 20:34 ` Andy Moreton
2021-08-13 6:29 ` Eli Zaretskii [this message]
2021-08-13 21:12 ` Andy Moreton
2021-08-14 5:54 ` Eli Zaretskii
2021-08-14 8:31 ` Andy Moreton
2021-08-14 8:57 ` Eli Zaretskii
2021-08-14 11:06 ` Andy Moreton
2021-08-14 11:33 ` Eli Zaretskii
2021-08-14 12:10 ` Andy Moreton
2021-08-14 12:36 ` Eli Zaretskii
2021-08-14 13:40 ` Andy Moreton
2021-08-14 14:43 ` Eli Zaretskii
2021-08-14 18:47 ` Andy Moreton
2021-08-15 6:07 ` Eli Zaretskii
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