[tarantool-patches] Re: [PATCH v1 1/1] lua: fix strange behaviour of tonumber64
- From: Alexander Turenko <alexander.turenko@xxxxxxxxxxxxx>
- To: Vladislav Shpilevoy <v.shpilevoy@xxxxxxxxxxxxx>
- Date: Mon, 16 Jul 2018 16:42:10 +0300
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.
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