https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97311
Bug ID: 97311
Summary: Bug in std::seed_seq::generate() when integer type has
more than 32 bits
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: kristian.spangsege at gmail dot com
Target Milestone: ---
As far as I can tell, std::seed_seq::generate() has a bug when _Type has more
than 32 bits.
The problem is that the following two additions can produce values greater
than, or equal to 2**32:
__begin[(__k + __p) % __n] += __r1;
__begin[(__k + __q) % __n] += __r2;
According to C++17 standard text, all operations must be performed modulo
2**32, so no generated value can be greater than, or equal to 2**32.
It seems that a possible fix would be to change those two lines to something
like this:
__begin[(__k + __p) % __n] = __detail::__mod<_Type, __detail::_Shift<_Type,
32>::__value>(__begin[(__k + __p) % __n] + __r1));
__begin[(__k + __q) % __n] = __detail::__mod<_Type, __detail::_Shift<_Type,
32>::__value>(__begin[(__k + __q) % __n] + __r2));
