[tarantool-patches] Re: [PATCH v1 1/1] lua: fix strange behaviour of tonumber64
Alexander Turenko
alexander.turenko at tarantool.org
Mon Jul 16 15:49:50 MSK 2018
Hi, Vlad!
That is interesting discussion. Hope you don't mind my participation.
WBR, Alexander Turenko.
On Mon, Jul 16, 2018 at 01:23:36PM +0300, Vladislav Shpilevoy wrote:
> Thanks for the patch! See 4 comments below.
>
> On 13/07/2018 14:21, Kirill Shcherbatov wrote:
> > Function tonumber64 has worked incorrectly with values less
> > than LLONG_MIN.
> > Now it works in the interval [LLONG_MIN, ULLONG_MAX] returning
> > nil otherwise.
> >
> > Closes #3466.
> > ---
> > Branch: https://github.com/tarantool/tarantool/compare/kshch/gh-3466-tonumber64-strange-behaviour
> > Issue: https://github.com/tarantool/tarantool/issues/3466
> >
> > src/lua/init.c | 6 +++++-
> > test/box/misc.result | 19 +++++++++++++++++++
> > test/box/misc.test.lua | 8 ++++++++
> > 3 files changed, 32 insertions(+), 1 deletion(-)
> >
> > diff --git a/src/lua/init.c b/src/lua/init.c
> > index 9a96030..4b5285d 100644
> > --- a/src/lua/init.c
> > +++ b/src/lua/init.c
> > @@ -222,7 +222,11 @@ lbox_tonumber64(struct lua_State *L)
> > if (argl == 0) {
> > lua_pushnil(L);
> > } else if (negative) {
> > - luaL_pushint64(L, -1 * (long long )result);
> > + if (result > -((unsigned long long )LLONG_MIN)) {
>
> 1. Please, do not enclose one-line bodies into {}.
>
> 2. How can you cast LLONG_MIN (that is negative) to the unsigned type?
>
Cast does not change bits. It is legal.
> 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
would be explicitly number-representation-dependent, so maybe it is
better.
> 4. Why the function is named to64, but we use non-explicitly sized types?
> I mean, why not to use uint64_t result, compare with INT64_MAX etc. According to
> the C standard, LLONG_MAX is not restricted with 64 bits.
>
Yep, it is better to use INT64_MIN from stdint.h.
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