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

[Tomasz Kaminski]

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'}
             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
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://gcc.gnu.org/pipermail/libstdc++/attachments/20250827/78c201c5/attachment-0001.htm>