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bug#32605: [w64] (random) never returns negative
From: |
Eli Zaretskii |
Subject: |
bug#32605: [w64] (random) never returns negative |
Date: |
Fri, 13 Aug 2021 09:29:41 +0300 |
> 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
- bug#32605: [w64] (random) never returns negative, Lars Ingebrigtsen, 2021/08/12
- bug#32605: [w64] (random) never returns negative, Eli Zaretskii, 2021/08/12
- bug#32605: [w64] (random) never returns negative, Andy Moreton, 2021/08/12
- bug#32605: [w64] (random) never returns negative,
Eli Zaretskii <=
- bug#32605: [w64] (random) never returns negative, Andy Moreton, 2021/08/13
- bug#32605: [w64] (random) never returns negative, Eli Zaretskii, 2021/08/14
- bug#32605: [w64] (random) never returns negative, Andy Moreton, 2021/08/14
- bug#32605: [w64] (random) never returns negative, Eli Zaretskii, 2021/08/14
- bug#32605: [w64] (random) never returns negative, Andy Moreton, 2021/08/14
- bug#32605: [w64] (random) never returns negative, Eli Zaretskii, 2021/08/14
- bug#32605: [w64] (random) never returns negative, Andy Moreton, 2021/08/14
- bug#32605: [w64] (random) never returns negative, Eli Zaretskii, 2021/08/14
- bug#32605: [w64] (random) never returns negative, Andy Moreton, 2021/08/14
- bug#32605: [w64] (random) never returns negative, Eli Zaretskii, 2021/08/14