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Date: Fri, 13 Aug 2021 14:40:08 +0300
From: Yura Sokolov <y.sokolov@postgrespro.ru>
To: Ranier Vilela <ranier.vf@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, PostgreSQL Hackers
 <pgsql-hackers@lists.postgresql.org>, Tomas Vondra
 <tomas.vondra@2ndquadrant.com>
Subject: Re: Bug in huge simplehash
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Ranier Vilela писал 2021-08-13 14:12:
> Em sex., 13 de ago. de 2021 às 07:15, Andres Freund
> <andres@anarazel.de> escreveu:
> 
>> Hi,
>> 
>> On 2021-08-13 12:44:17 +0300, Yura Sokolov wrote:
>>> Andres Freund писал 2021-08-13 12:21:
>>>> Any chance you'd write a test for simplehash with such huge
>> amount of
>>>> values? It'd require a small bit of trickery to be practical. On
>> systems
>>>> with MAP_NORESERVE it should be feasible.
>>> 
>>> Which way C structures should be tested in postgres?
>>> dynahash/simplhash - do they have any tests currently?
>>> I'll do if hint is given.
>> 
>> We don't have a great way right now :(. I think the best is to have
>> a
>> SQL callable function that uses some API. See e.g. test_atomic_ops()
>> et
>> al in src/test/regress/regress.c
>> 
>>>> >  static inline void
>>>> > -SH_COMPUTE_PARAMETERS(SH_TYPE * tb, uint32 newsize)
>>>> > +SH_COMPUTE_PARAMETERS(SH_TYPE * tb, uint64 newsize)
>>>> >  {
>>>> >   uint64          size;
>>>> >
>>>> > @@ -322,11 +322,7 @@ SH_COMPUTE_PARAMETERS(SH_TYPE * tb,
>> uint32
>>>> > newsize)
>>>> >
>>>> >   /* now set size */
>>>> >   tb->size = size;
>>>> > -
>>>> > - if (tb->size == SH_MAX_SIZE)
>>>> > -         tb->sizemask = 0;
>>>> > - else
>>>> > -         tb->sizemask = tb->size - 1;
>>>> > + tb->sizemask = (uint32)(size - 1);
>>>> 
>>>> ISTM using ~0 would be nicer here?
>>> 
>>> I don't think so.
>>> To be rigid it should be `~(uint32)0`.
>>> But I believe it doesn't differ from `tb->sizemask = (uint32)(size
>> - 1)`
>>> that is landed with patch, therefore why `if` is needed?
>> 
>> Personally I find it more obvious to understand the intended
>> behaviour
>> with ~0 (i.e. all bits set) than with a width truncation.
> 
> https://godbolt.org/z/57WcjKqMj
> The generated code is identical.

I believe, you mean https://godbolt.org/z/qWzE1ePTE

> regards,
> Ranier Vilela