[PATCH] 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:
  static_cast<unsigned char>(static_cast<unsigned int>(-_M_value))).
The static casts are required to avoid undefined behavior from signed overflow.

The operator<=(partial_order, zero_type) 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 (__cmp_cat::__utype): Define.
	(_Ncmp::_Unordered): Change the value to minimum value of
	signed char.
	(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.
---
Tested on x86_64-linux. Testing on powerpc64le.
OK for trunk when test passes?

 libstdc++-v3/libsupc++/compare | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/libsupc++/compare b/libstdc++-v3/libsupc++/compare
index 82b5c53139c..ae3a1c5e612 100644
--- a/libstdc++-v3/libsupc++/compare
+++ b/libstdc++-v3/libsupc++/compare
@@ -53,10 +53,14 @@ namespace std _GLIBCXX_VISIBILITY(default)
   namespace __cmp_cat
   {
     using type = signed char;
+    using __utype = unsigned char;
 
     enum class _Ord : type { equivalent = 0, less = -1, greater = 1 };
 
-    enum class _Ncmp : type { _Unordered = 2 };
+    enum class _Ncmp : type { 
+      // minimum value of signed char, wraparound negation is no-op
+      _Unordered = static_cast<type>(static_cast<__utype>(1u << (__CHAR_BIT__ - 1)))
+    };
 
     struct __unspec
     {
@@ -66,7 +70,7 @@ 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 +86,15 @@ namespace std _GLIBCXX_VISIBILITY(default)
     friend class weak_ordering;
     friend class strong_ordering;
 
+    [[__gnu__::__always_inline__]]
+    constexpr __cmp_cat::type
+    _M_reverse() const
+    {
+      return static_cast<__cmp_cat::type>(
+	       static_cast<__cmp_cat::__utype>(
+		 static_cast<unsigned int>(-_M_value)));
+    }
+
   public:
     // valid values
     static const partial_ordering less;
@@ -112,12 +125,12 @@ 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 +145,12 @@ 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
-- 
2.50.1