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From: Alexander Turenko <alexander.turenko@tarantool.org>
To: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Cc: Kirill Shcherbatov <kshcherbatov@tarantool.org>,
	tarantool-patches@freelists.org
Subject: [tarantool-patches] Re: [PATCH v1 1/1] lua: fix strange behaviour of tonumber64
Date: Mon, 16 Jul 2018 16:42:10 +0300	[thread overview]
Message-ID: <20180716134210.zp3tckp3gcgchm4q@tkn_work_nb> (raw)
In-Reply-To: <ac001997-5bfb-443d-5f21-ac345a29bd7d@tarantool.org>

> > 
> > > 3. Why not 'result > LLONG_MAX'? As I understand, abs(LLONG_MAX) == abs(LLONG_MIN),
> > > it is not? (http://www.cplusplus.com/reference/climits/)
> > > 
> > 
> > No, LLONG_MAX is 2^63-1, but LLONG_MIN is -2^63. We want to compare
> > result with 2^63. We are trying to do so in platform-independent way
> > (hovewer unsiged unary nimus equivalence with signed one is likely
> > two-complement number representation property and can be violated on
> > other platforms).
> > 
> > Are you think we should introduce our own constant
> > 9223372036854775808ULL (2^63) and avoid that complex assumptions set? It
> 
> Ultimately no. We should not invent the constants.
> 
> > would be explicitly number-representation-dependent, so maybe it is
> > better.
> 
> Ok. Logically we want an error on -result < INT64_MIN, right?
> It is the same as result > -INT64_MIN. But we can not say
> -INT64_MIN because abs(INT64_MIN) > INT64_MAX, yes?
> 

Yes.

> Then lets rephrase the comparison:
> 
>     result > -INT64_MIN
>            |
>            v
>   result + 1 >= -INT64_MIN
>            |
>            v
>     result >= -INT64_MIN - 1
>            |
>            v
>    result >= -(INT64_MIN + 1) <- that is the solution.
> 
> As I understand, -(INT64_MIN + 1) is exactly 2^63 - 1 and
> fits in int64, right?

2nd step should be result - 1 >= -INT64_MIN, so not it is not the
decision. Overflow is unavoidable while we are trying to operate within
the signed type.

WBR, Alexander Turenko.

  reply	other threads:[~2018-07-16 13:42 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-13 11:21 [tarantool-patches] " Kirill Shcherbatov
2018-07-16 10:23 ` [tarantool-patches] " Vladislav Shpilevoy
2018-07-16 12:49   ` Alexander Turenko
2018-07-16 13:15     ` Vladislav Shpilevoy
2018-07-16 13:42       ` Alexander Turenko [this message]
2018-07-16 13:55         ` Vladislav Shpilevoy
2018-07-16 14:09           ` Alexander Turenko
2018-07-16 16:52             ` Kirill Shcherbatov
2018-07-17  9:35               ` Vladislav Shpilevoy
2018-07-17 11:27                 ` Alexander Turenko
2018-07-17 12:03                   ` Kirill Shcherbatov
2018-07-17 21:48                     ` Vladislav Shpilevoy
2018-07-19 10:46 ` Kirill Yukhin

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