[Bug libstdc++/104945] New: std::hash<std::uint64_t> ignores the top 32 bits when size_t is 32 bit

[Bug libstdc++/104945] New: std::hash<std::uint64_t> ignores the top 32 bits when size_t is 32 bit

[pdimov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104945

            Bug ID: 104945
           Summary: std::hash<std::uint64_t> ignores the top 32 bits when
                    size_t is 32 bit
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pdimov at gmail dot com
  Target Milestone: ---

A benchmark in Boost.Unordered uses uint64_t keys of the form `i << 40`:

https://github.com/boostorg/unordered/blob/33f81fd49039bccd1aa3dfd5a29ef6073b93009c/benchmark/uint64.cpp#L65

which leads to pathological behavior with std::unordered_map in 32 bit mode,
because std::hash<std::uint64_t> ignores the top 32 bits:

https://godbolt.org/z/PncKbT7aq

I'm not entirely sure whether this would be considered a bug, but decided that
it's worth reporting. Ideally, std::hash ought to take into account all bits of
the integral input, instead of truncating it to size_t.

[Bug libstdc++/104945] std::hash<std::uint64_t> ignores the top 32 bits when size_t is 32 bit

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104945

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Yes this is definitely an issue I think:
  // Explicit specializations for integer types.
#define _Cxx_hashtable_define_trivial_hash(_Tp)         \
  template<>                                            \
    struct hash<_Tp> : public __hash_base<size_t, _Tp>  \
    {                                                   \
      size_t                                            \
      operator()(_Tp __val) const noexcept              \
      { return static_cast<size_t>(__val); }            \
    };

....
  /// Explicit specialization for long long.
  _Cxx_hashtable_define_trivial_hash(long long)

....
  /// Explicit specialization for unsigned long long.
  _Cxx_hashtable_define_trivial_hash(unsigned long long)


With no check on size of size_t vs long long.

[Bug libstdc++/104945] std::hash<std::uint64_t> ignores the top 32 bits when size_t is 32 bit

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104945

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-03-16
           Keywords|                            |ABI
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=98781
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1