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libstdc++: Fix up constexpr std::char_traits<char>::compare [PR99181]

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Because of LWG 467, std::char_traits<char>::lt compares the values
cast to unsigned char rather than char, so even when char is signed
we get unsigned comparision.  std::char_traits<char>::compare uses
__builtin_memcmp and that works the same, but during constexpr evaluation
we were calling __gnu_cxx::char_traits<char_type>::compare.  As
char_traits::lt is not virtual, __gnu_cxx::char_traits<char_type>::compare
used __gnu_cxx::char_traits<char_type>::lt rather than
std::char_traits<char>::lt and thus compared chars as signed if char is
signed.
This change fixes it by inlining __gnu_cxx::char_traits<char_type>::compare
into std::char_traits<char>::compare by hand, so that it calls the right
lt method.

2021-02-23  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/99181
	* include/bits/char_traits.h (char_traits<char>::compare): For
	constexpr evaluation don't call
	__gnu_cxx::char_traits<char_type>::compare but do the comparison loop
	directly.

	* testsuite/21_strings/char_traits/requirements/char/99181.cc: New
	test.
This commit is contained in:
Jakub Jelinek 2021-02-23 09:30:18 +01:00
parent 7e647d71d5
commit efa64fcce1
2 changed files with 48 additions and 1 deletions
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9
libstdc++-v3/include/bits/char_traits.h
View file

@ -349,7 +349,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
if (__builtin_constant_p(__n)
&& __constant_char_array_p(__s1, __n)
&& __constant_char_array_p(__s2, __n))
return __gnu_cxx::char_traits<char_type>::compare(__s1, __s2, __n);
{
for (size_t __i = 0; __i < __n; ++__i)
if (lt(__s1[__i], __s2[__i]))
return -1;
else if (lt(__s2[__i], __s1[__i]))
return 1;
return 0;
}
#endif
return __builtin_memcmp(__s1, __s2, __n);
}

40
libstdc++-v3/testsuite/21_strings/char_traits/requirements/char/99181.cc Normal file
View file

@ -0,0 +1,40 @@
// { dg-options "-std=gnu++17" }
// { dg-do run { target c++17 } }
// Copyright (C) 2021 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/>.
#include <string>
#include <testsuite_hooks.h>
void test01()
{
const char *a = "\x7f";
const char *b = "\x80";
int c = std::char_traits<char>::compare(a, b, 2);
constexpr int d = std::char_traits<char>::compare("\x7f", "\x80", 2);
VERIFY( c && (c < 0) == (static_cast<unsigned char>(a[0])
< static_cast<unsigned char>(b[0])) );
VERIFY( d && (c < 0) == (d < 0) );
}
int main()
{
test01();
return 0;
}