libstdc++: Fix three-way comparison for std::array [PR 96851]

The spaceship operator for std::array uses memcmp when the
__is_byte<value_type> trait is true, but memcmp isn't usable in
constexpr contexts. Also, memcmp should only be used for unsigned byte
types, because it gives the wrong answer for signed chars with negative
values.

We can simply check std::is_constant_evaluated() so that we don't use
memcmp during constant evaluation.

To fix the problem of using memcmp for inappropriate types, this patch
adds new __is_memcmp_ordered and __is_memcmp_ordered_with traits. These
say whether using memcmp will give the right answer for ordering
operations such as lexicographical_compare and three-way comparisons.
The new traits can be used in several places, and can also be used to
implement my suggestion in PR 93059 comment 37 to use memcmp for
unsigned integers larger than one byte on big endian targets.

libstdc++-v3/ChangeLog:

	PR libstdc++/96851
	* include/bits/cpp_type_traits.h (__is_memcmp_ordered):
	New trait that says if memcmp can be used for ordering.
	(__is_memcmp_ordered_with): Likewise, for two types.
	* include/bits/deque.tcc (__lex_cmp_dit): Use new traits
	instead of __is_byte and __numeric_traits.
	(__lexicographical_compare_aux1): Likewise.
	* include/bits/ranges_algo.h (__lexicographical_compare_fn):
	Likewise.
	* include/bits/stl_algobase.h (__lexicographical_compare_aux1)
	(__is_byte_iter): Likewise.
	* include/std/array (operator<=>): Likewise. Only use memcmp
	when std::is_constant_evaluated() is false.
	* testsuite/23_containers/array/comparison_operators/96851.cc:
	New test.
	* testsuite/23_containers/array/tuple_interface/get_neg.cc:
	Adjust dg-error line numbers.

7 files changed, 200 insertions, 27 deletions

diff --git a/libstdc++-v3/include/bits/cpp_type_traits.h b/libstdc++-v3/include/bits/cpp_type_traits.h
index 979ad9c..b48d1ad 100644
--- a/libstdc++-v3/include/bits/cpp_type_traits.h
+++ b/libstdc++-v3/include/bits/cpp_type_traits.h
@@ -482,6 +482,66 @@ __INT_N(__GLIBCXX_TYPE_INT_N_3)
     : __is_nonvolatile_trivially_copyable<_Tp>
     { };
 
+  // Whether memcmp can be used to determine ordering for a type
+  // e.g. in std::lexicographical_compare or three-way comparisons.
+  // True for unsigned integer-like types where comparing each byte in turn
+  // as an unsigned char yields the right result. This is true for all
+  // unsigned integers on big endian targets, but only unsigned narrow
+  // character types (and std::byte) on little endian targets.
+  template<typename _Tp, bool _TreatAsBytes =
+#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+	__is_integer<_Tp>::__value
+#else
+	__is_byte<_Tp>::__value
+#endif
+    >
+    struct __is_memcmp_ordered
+    {
+      static const bool __value = _Tp(-1) > _Tp(1); // is unsigned
+    };
+
+  template<typename _Tp>
+    struct __is_memcmp_ordered<_Tp, false>
+    {
+      static const bool __value = false;
+    };
+
+  // Whether two types can be compared using memcmp.
+  template<typename _Tp, typename _Up, bool = sizeof(_Tp) == sizeof(_Up)>
+    struct __is_memcmp_ordered_with
+    {
+      static const bool __value = __is_memcmp_ordered<_Tp>::__value
+	&& __is_memcmp_ordered<_Up>::__value;
+    };
+
+  template<typename _Tp, typename _Up>
+    struct __is_memcmp_ordered_with<_Tp, _Up, false>
+    {
+      static const bool __value = false;
+    };
+
+#if __cplusplus >= 201703L
+#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+  // std::byte is not an integer, but it can be compared using memcmp.
+  template<>
+    struct __is_memcmp_ordered<std::byte, false>
+    { static constexpr bool __value = true; };
+#endif
+
+  // std::byte can only be compared to itself, not to other types.
+  template<>
+    struct __is_memcmp_ordered_with<std::byte, std::byte, true>
+    { static constexpr bool __value = true; };
+
+  template<typename _Tp, bool _SameSize>
+    struct __is_memcmp_ordered_with<_Tp, std::byte, _SameSize>
+    { static constexpr bool __value = false; };
+
+  template<typename _Up, bool _SameSize>
+    struct __is_memcmp_ordered_with<std::byte, _Up, _SameSize>
+    { static constexpr bool __value = false; };
+#endif
+
   //
   // Move iterator type
   //
diff --git a/libstdc++-v3/include/bits/deque.tcc b/libstdc++-v3/include/bits/deque.tcc
index 7d1ec86..651ae70 100644
--- a/libstdc++-v3/include/bits/deque.tcc
+++ b/libstdc++-v3/include/bits/deque.tcc
@@ -1269,9 +1269,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
 	const _Tp2* __first2, const _Tp2* __last2)
     {
       const bool __simple =
-	(__is_byte<_Tp1>::__value && __is_byte<_Tp2>::__value
-	 && !__gnu_cxx::__numeric_traits<_Tp1>::__is_signed
-	 && !__gnu_cxx::__numeric_traits<_Tp2>::__is_signed
+	(__is_memcmp_ordered_with<_Tp1, _Tp2>::__value
 	 && __is_pointer<_Ptr>::__value
 #if __cplusplus > 201703L && __cpp_lib_concepts
 	 // For C++20 iterator_traits<volatile T*>::value_type is non-volatile
@@ -1327,9 +1325,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
 		_GLIBCXX_STD_C::_Deque_iterator<_Tp2, _Ref2, _Ptr2> __last2)
     {
       const bool __simple =
-	(__is_byte<_Tp1>::__value && __is_byte<_Tp2>::__value
-	 && !__gnu_cxx::__numeric_traits<_Tp1>::__is_signed
-	 && !__gnu_cxx::__numeric_traits<_Tp2>::__is_signed
+	(__is_memcmp_ordered_with<_Tp1, _Tp2>::__value
 	 && __is_pointer<_Ptr1>::__value
 	 && __is_pointer<_Ptr2>::__value
 #if __cplusplus > 201703L && __cpp_lib_concepts
diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
index 94ca7b6..a553965 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -3475,10 +3475,7 @@ namespace ranges
 		// This condition is consistent with the one in
 		// __lexicographical_compare_aux in <bits/stl_algobase.h>.
 		constexpr bool __use_memcmp
-		  = (__is_byte<_ValueType1>::__value
-		     && __is_byte<_ValueType2>::__value
-		     && !__gnu_cxx::__numeric_traits<_ValueType1>::__is_signed
-		     && !__gnu_cxx::__numeric_traits<_ValueType2>::__is_signed
+		  = (__is_memcmp_ordered_with<_ValueType1, _ValueType2>::__value
 		     && __ptr_to_nonvolatile<_Iter1>
 		     && __ptr_to_nonvolatile<_Iter2>
 		     && (is_same_v<_Comp, ranges::less>
diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index ff5b450..3e9ec32 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -1368,9 +1368,7 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
       typedef typename iterator_traits<_II1>::value_type _ValueType1;
       typedef typename iterator_traits<_II2>::value_type _ValueType2;
       const bool __simple =
-	(__is_byte<_ValueType1>::__value && __is_byte<_ValueType2>::__value
-	 && !__gnu_cxx::__numeric_traits<_ValueType1>::__is_signed
-	 && !__gnu_cxx::__numeric_traits<_ValueType2>::__is_signed
+	(__is_memcmp_ordered_with<_ValueType1, _ValueType2>::__value
 	 && __is_pointer<_II1>::__value
 	 && __is_pointer<_II2>::__value
 #if __cplusplus > 201703L && __cpp_lib_concepts
@@ -1785,8 +1783,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
   // or std::byte, suitable for comparison by memcmp.
   template<typename _Iter>
     concept __is_byte_iter = contiguous_iterator<_Iter>
-      && __is_byte<iter_value_t<_Iter>>::__value != 0
-      && !__gnu_cxx::__numeric_traits<iter_value_t<_Iter>>::__is_signed;
+      && __is_memcmp_ordered<iter_value_t<_Iter>>::__value;
 
   // Return a struct with two members, initialized to the smaller of x and y
   // (or x if they compare equal) and the result of the comparison x <=> y.
diff --git a/libstdc++-v3/include/std/array b/libstdc++-v3/include/std/array
index f7b0dca..3bb6f48 100644
--- a/libstdc++-v3/include/std/array
+++ b/libstdc++-v3/include/std/array
@@ -258,16 +258,20 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
     constexpr __detail::__synth3way_t<_Tp>
     operator<=>(const array<_Tp, _Nm>& __a, const array<_Tp, _Nm>& __b)
     {
-      if constexpr (_Nm && __is_byte<_Tp>::__value)
-	return __builtin_memcmp(__a.data(), __b.data(), _Nm) <=> 0;
-      else
+#ifdef __cpp_lib_is_constant_evaluated
+      if constexpr (_Nm && __is_memcmp_ordered<_Tp>::__value)
+	if (!std::is_constant_evaluated())
+	  {
+	    constexpr size_t __n = _Nm * sizeof(_Tp);
+	    return __builtin_memcmp(__a.data(), __b.data(), __n) <=> 0;
+	  }
+#endif
+
+      for (size_t __i = 0; __i < _Nm; ++__i)
 	{
-	  for (size_t __i = 0; __i < _Nm; ++__i)
-	    {
-	      auto __c = __detail::__synth3way(__a[__i], __b[__i]);
-	      if (__c != 0)
-		return __c;
-	    }
+	  auto __c = __detail::__synth3way(__a[__i], __b[__i]);
+	  if (__c != 0)
+	    return __c;
 	}
       return strong_ordering::equal;
     }
diff --git a/libstdc++-v3/testsuite/23_containers/array/comparison_operators/96851.cc b/libstdc++-v3/testsuite/23_containers/array/comparison_operators/96851.cc
new file mode 100644
index 0000000..b2a464b
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/array/comparison_operators/96851.cc
@@ -0,0 +1,119 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <array>
+#include <testsuite_hooks.h>
+
+template<typename T>
+constexpr auto
+cmp_array(T t)
+{
+  std::array<T, 2> a{ T{1}, t };
+  std::array<T, 2> b{ T{1}, T{2} };
+  return a <=> b;
+}
+
+void
+test01()
+{
+  static_assert( cmp_array((unsigned char)1) < 0 );
+  static_assert( cmp_array((unsigned char)2) == 0 );
+  static_assert( cmp_array((unsigned char)3) > 0 );
+
+  static_assert( cmp_array((signed char)-1) < 0 );
+  static_assert( cmp_array((signed char)2) == 0 );
+  static_assert( cmp_array((signed char)3) > 0 );
+
+  static_assert( cmp_array(std::byte{1}) < 0 );
+  static_assert( cmp_array(std::byte{2}) == 0 );
+  static_assert( cmp_array(std::byte{3}) > 0 );
+
+  static_assert( cmp_array((unsigned int)1) < 0 );
+  static_assert( cmp_array((unsigned int)2) == 0 );
+  static_assert( cmp_array((unsigned int)333) > 0 );
+  static_assert( cmp_array((unsigned int)4444) > 0 );
+  static_assert( cmp_array((unsigned int)55555) > 0 );
+  static_assert( cmp_array((unsigned int)0x66666666) > 0 );
+
+  static_assert( cmp_array((signed int)-1) < 0 );
+  static_assert( cmp_array((signed int)2) == 0 );
+  static_assert( cmp_array((signed int)333) > 0 );
+  static_assert( cmp_array((signed int)-4444) < 0 );
+  static_assert( cmp_array((signed int)55555) > 0 );
+  static_assert( cmp_array((signed int)-0x66666666) < 0 );
+}
+
+void
+test02()
+{
+  unsigned char uc = 1;
+  VERIFY( cmp_array(uc) < 0 );
+  uc = 2;
+  VERIFY( cmp_array(uc) == 0 );
+  uc = 3;
+  VERIFY( cmp_array(uc) > 0 );
+
+  signed char sc = -1;
+  VERIFY( cmp_array(sc) < 0 );
+  sc = 2;
+  VERIFY( cmp_array(sc) == 0 );
+  sc = 3;
+  VERIFY( cmp_array(sc) > 0 );
+
+  std::byte b{1};
+  VERIFY( cmp_array(b) < 0 );
+  b = std::byte{2};
+  VERIFY( cmp_array(b) == 0 );
+  b = std::byte{3};
+  VERIFY( cmp_array(b) > 0 );
+
+  unsigned int ui = 1;
+  VERIFY( cmp_array(ui) < 0 );
+  ui = 2;
+  VERIFY( cmp_array(ui) == 0 );
+  ui = 333;
+  VERIFY( cmp_array(ui) > 0 );
+  ui = 4444;
+  VERIFY( cmp_array(ui) > 0 );
+  ui = 555555;
+  VERIFY( cmp_array(ui) > 0 );
+  ui = 0x66666666;
+  VERIFY( cmp_array(ui) > 0 );
+
+  signed int si = -1;
+  VERIFY( cmp_array(si) < 0 );
+  si = 2;
+  VERIFY( cmp_array(si) == 0 );
+  si = 333;
+  VERIFY( cmp_array(si) > 0 );
+  si = -4444;
+  VERIFY( cmp_array(si) < 0 );
+  si = 555555;
+  VERIFY( cmp_array(si) > 0 );
+  si = -0x66666666;
+  VERIFY( cmp_array(si) < 0 );
+}
+
+int
+main()
+{
+  test01();
+  test02();
+}
diff --git a/libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc b/libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc
index 6072d07..a264031 100644
--- a/libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc
@@ -27,6 +27,6 @@ int n1 = std::get<1>(a);
 int n2 = std::get<1>(std::move(a));
 int n3 = std::get<1>(ca);
 
-// { dg-error "static assertion failed" "" { target *-*-* } 336 }
-// { dg-error "static assertion failed" "" { target *-*-* } 345 }
-// { dg-error "static assertion failed" "" { target *-*-* } 353 }
+// { dg-error "static assertion failed" "" { target *-*-* } 340 }
+// { dg-error "static assertion failed" "" { target *-*-* } 349 }
+// { dg-error "static assertion failed" "" { target *-*-* } 357 }