GNU bug report logs - #32605
[w64] (random) never returns negative

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From: Eli Zaretskii <eliz <at> gnu.org>
To: Andy Moreton <andrewjmoreton <at> gmail.com>
Cc: 32605 <at> debbugs.gnu.org
Subject: bug#32605: [w64] (random) never returns negative
Date: Fri, 13 Aug 2021 09:29:41 +0300

> From: Andy Moreton <andrewjmoreton <at> 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

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