[PATCH v2] libsupc++: Change _Unordered comparison value to minimum value of signed char.

[Jonathan Wakely]

On Wed, 27 Aug 2025 at 11:36, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
>
> Again, pretty printers needs to get updated:
> iff --git a/libstdc++-v3/python/libstdcxx/v6/printers.py b/libstdc++-v3/python/libstdcxx/v6/printers.py
> index 5f5963cb595..cf510433fd4 100644
> --- a/libstdc++-v3/python/libstdcxx/v6/printers.py
> +++ b/libstdc++-v3/python/libstdcxx/v6/printers.py
> @@ -1749,7 +1749,7 @@ class StdCmpCatPrinter(printer_base):
>          if self._typename == 'strong_ordering' and self._val == 0:
>              name = 'equal'
>          else:
> -            names = {2: 'unordered', -1: 'less', 0: 'equivalent', 1: 'greater'}
> +            names = {-128: 'unordered', -1: 'less', 0: 'equivalent', 1: 'greater'}

Can we keep 2 in the map as well, so that objects built with older
versions of GCC can still be debugged with new printers.

i.e.
# GCC 10-15 used 2 for unordered
names = {-128: 'unordered', -1: 'less', 0: 'equivalent', 1: 'greater',
2: 'unordered'}



>              name = names[int(self._val)]
>          return 'std::{}::{}'.format(self._typename, name)
>
>
> On Wed, Aug 27, 2025 at 12:05 PM Tomasz Kamiński <tkaminsk@redhat.com> wrote:
>>
>> For any minimum value of a signed type, its negation (with wraparound) results
>> in the same value, behaving like zero. Representing the unordered result with
>> this minimum value, along with 0 for equal, 1 for greater, and -1 for less
>> in partial_ordering, allows its value to be reversed using unary negation.
>>
>> The operator<=(partial_order, 0) now checks if the reversed value is positive.
>> This works correctly because the unordered value remains unchanged and thus
>> negative.
>>
>> libstdc++-v3/ChangeLog:
>>
>>         * libsupc++/compare (_Ncmp::_Unordered): Rename and change the value
>>         to minimum value of signed char.
>>         (_Ncomp::unordered): Renamed from _Unordered, the name is reserved
>>         by partial_ordered::unordered.
>>         (partial_ordering::_M_reverse()): Define.
>>         (operator<=(partial_ordering, __cmp_cat::__unspec))
>>         (operator>=(__cmp_cat::__unspec, partial_ordering)): Implemented
>>         in terms of negated _M_value.
>>         (operator>=(partial_ordering, __cmp_cat::__unspec))
>>         (operator<=(__cmp_cat::__unspec, partial_ordering)): Directly
>>         compare _M_value, as unordered value is negative.
>>         (partial_ordering::unordered): Handle _Ncmp::unoredred rename.
>> ---
>> Changes in v3:
>> * rename and simplify defintion of _Ncmp::unordered
>> * simplify defintion of _M_reverse
>>
>> Testing on x86_64-linux and powerpc64le-linux. OK for trunk when test
>> passes?
>>
>>  libstdc++-v3/libsupc++/compare | 22 +++++++++++++++-------
>>  1 file changed, 15 insertions(+), 7 deletions(-)
>>
>> diff --git a/libstdc++-v3/libsupc++/compare b/libstdc++-v3/libsupc++/compare
>> index 82b5c53139c..2624fa9144f 100644
>> --- a/libstdc++-v3/libsupc++/compare
>> +++ b/libstdc++-v3/libsupc++/compare
>> @@ -56,7 +56,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
>>
>>      enum class _Ord : type { equivalent = 0, less = -1, greater = 1 };
>>
>> -    enum class _Ncmp : type { _Unordered = 2 };
>> +    enum class _Ncmp : type { unordered = -__SCHAR_MAX__ - 1 };
>>
>>      struct __unspec
>>      {
>> @@ -66,7 +66,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
>>
>>    class partial_ordering
>>    {
>> -    // less=0xff, equiv=0x00, greater=0x01, unordered=0x02
>> +    // less=0xff, equiv=0x00, greater=0x01, unordered=0x80
>>      __cmp_cat::type _M_value;
>>
>>      constexpr explicit
>> @@ -82,6 +82,14 @@ namespace std _GLIBCXX_VISIBILITY(default)
>>      friend class weak_ordering;
>>      friend class strong_ordering;
>>
>> +    [[__gnu__::__always_inline__]]
>> +    constexpr __cmp_cat::type
>> +    _M_reverse() const
>> +    {
>> +      // leaves _Ncmp::unordered unchanged
>> +      return static_cast<__cmp_cat::type>(-_M_value);
>> +    }
>> +
>>    public:
>>      // valid values
>>      static const partial_ordering less;
>> @@ -112,12 +120,12 @@ namespace std _GLIBCXX_VISIBILITY(default)
>>      [[nodiscard]]
>>      friend constexpr bool
>>      operator<=(partial_ordering __v, __cmp_cat::__unspec) noexcept
>> -    { return __v._M_value <= 0; }
>> +    { return __v._M_reverse() >= 0; }
>>
>>      [[nodiscard]]
>>      friend constexpr bool
>>      operator>=(partial_ordering __v, __cmp_cat::__unspec) noexcept
>> -    { return __cmp_cat::type(__v._M_value & 1) == __v._M_value; }
>> +    { return __v._M_value >= 0; }
>>
>>      [[nodiscard]]
>>      friend constexpr bool
>> @@ -132,12 +140,12 @@ namespace std _GLIBCXX_VISIBILITY(default)
>>      [[nodiscard]]
>>      friend constexpr bool
>>      operator<=(__cmp_cat::__unspec, partial_ordering __v) noexcept
>> -    { return __cmp_cat::type(__v._M_value & 1) == __v._M_value; }
>> +    { return 0 <= __v._M_value; }
>>
>>      [[nodiscard]]
>>      friend constexpr bool
>>      operator>=(__cmp_cat::__unspec, partial_ordering __v) noexcept
>> -    { return 0 >= __v._M_value; }
>> +    { return 0 <= __v._M_reverse(); }
>>
>>      [[nodiscard]]
>>      friend constexpr partial_ordering
>> @@ -166,7 +174,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
>>    partial_ordering::greater(__cmp_cat::_Ord::greater);
>>
>>    inline constexpr partial_ordering
>> -  partial_ordering::unordered(__cmp_cat::_Ncmp::_Unordered);
>> +  partial_ordering::unordered(__cmp_cat::_Ncmp::unordered);
>>
>>    class weak_ordering
>>    {
>> --
>> 2.50.1
>>