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.
Unlike the trunk commit this was backported from, this commit for the
branch doesn't extend the memcmp optimisations to all unsigned integers
on big endian targets. Only narrow character types and std::byte will
use memcmp.
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/ranges_algo.h (__lexicographical_compare_fn):
Use new traits instead of __is_byte and __numeric_traits.
* 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.
(cherry picked from commit
2f983fa69005b603ea1758a013b4134d5b0f24a8)
: __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 narrow character types (and std::byte).
+ template<typename _Tp, bool _TreatAsBytes = __is_byte<_Tp>::__value>
+ 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
+ // 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
//
// 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>
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
// 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.
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;
}
--- /dev/null
+// 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();
+}
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 }
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