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bug#32605: [w64] (random) never returns negative
From: |
Andy Moreton |
Subject: |
bug#32605: [w64] (random) never returns negative |
Date: |
Sat, 14 Aug 2021 13:10:08 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (windows-nt) |
On Sat 14 Aug 2021, Eli Zaretskii wrote:
>> From: Andy Moreton <andrewjmoreton@gmail.com>
>> Date: Sat, 14 Aug 2021 12:06:00 +0100
>>
>> >> I'm not an expert on random numbers either, and your efforts are not an
>> >> annoyance, but I am puzzled why you so strongly prize performance over
>> >> correctness in this instance.
>> >
>> > Because I have no idea how important the "correctness" part is, or
>> > why. OTOH, this stuff, when used, tends to be in the inner loops, so
>> > performance matters.
>>
>> I doubt anyone expects cryptographic quality randomness or any given
>> statistical distribution from such a general purpose routine, but they
>> have a reasonable expectation that the results from 'get_random' do not
>> have stuck bits that are always non-random.
>>
>> In which case perhaps the solution is to change the RAND_BITS logic
>> in sysdep.c on Windows to override the RAND_BITS definition:
>>
>> + #ifdef WINDOWSNT
>> + /* Use w32.c replacement for random(). */
>> + # define RAND_BITS 15
>> + #endif
>>
>> #ifndef RAND_BITS
>> # ifdef HAVE_RANDOM
>> # define RAND_BITS 31
>> # else /* !HAVE_RANDOM */
>> ...
>> #endif
>>
>> ..and then in w32.c make 'random' return the 15bit value from
>> 'rand_as183':
>
> Why not keep the 30 bits we produce today on 32-bit builds?
For 32bit builds (FIXNUM_BITS is 30), either:
a) define RAND_BITS to 30, 'random' calls 'rand_as183' twice.
'get_random' needs 1 call to 'random' (total 2 calls of 'rand_as183').
b) define RAND_BITS to 15, 'random' calls 'rand_as183' once.
'get_random' needs 2 calls to 'random' (total 2 calls of 'rand_as183').
For 64bit builds (FIXNUM_BITS is 62), either:
a) define RAND_BITS to 30, 'random' calls 'rand_as183' twice.
'get_random' needs 3 calls to 'random' (total 6 calls of 'rand_as183').
b) define RAND_BITS to 15, 'random' calls 'rand_as183' once.
'get_random' needs 4 calls to 'random' (total 4 calls of 'rand_as183').
On 32bit builds both options are roughly equivalent.
On 64bit builds option (b) is better as option (a) does unnecessary work.
AndyM
- 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, 2021/08/13
- 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 <=
- 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, 2021/08/15