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

[Tomasz Kamiński]

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