From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 2181) id 1A7F63858033; Wed, 5 Jan 2022 22:07:13 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 1A7F63858033 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="utf-8" From: Jonathan Wakely To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r11-9441] libstdc++: Fix std::char_traits::move for constexpr X-Act-Checkin: gcc X-Git-Author: Jonathan Wakely X-Git-Refname: refs/heads/releases/gcc-11 X-Git-Oldrev: 0bc256f8e1ae6db423c167f15ffdd126fd8199f9 X-Git-Newrev: 226210894e53259940a8e1453125e37c65299ba4 Message-Id: <20220105220713.1A7F63858033@sourceware.org> Date: Wed, 5 Jan 2022 22:07:13 +0000 (GMT) X-BeenThere: libstdc++-cvs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libstdc++-cvs mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 05 Jan 2022 22:07:13 -0000 https://gcc.gnu.org/g:226210894e53259940a8e1453125e37c65299ba4 commit r11-9441-g226210894e53259940a8e1453125e37c65299ba4 Author: Jonathan Wakely Date: Thu Nov 18 12:39:20 2021 +0000 libstdc++: Fix std::char_traits::move for constexpr The constexpr branch in __gnu_cxx::char_traits::move compares the string arguments to see if they overlap, but relational comparisons between unrelated pointers are not core constant expressions. I want to replace the comparisons with a loop using pointer equality to determine whether the end of the source string is in the destination string. However, that doesn't work with GCC, due to PR c++/89074 so allocate a temporary buffer instead and copy out into that first, so that overlapping source and destination don't matter. The allocation isn't supported by the current Intel icc so use the loop as a fallback. libstdc++-v3/ChangeLog: * include/bits/char_traits.h (__gnu_cxx::char_traits::move): Do not compare unrelated pointers during constant evaluation. * testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc: Improve tests for char_traits::move. (cherry picked from commit ca243ada71656651a8753e88164a1f0f019be1c3) Diff: --- libstdc++-v3/include/bits/char_traits.h | 40 ++++++++++++++++++---- .../requirements/constexpr_functions_c++20.cc | 23 +++++++++++-- 2 files changed, 54 insertions(+), 9 deletions(-) diff --git a/libstdc++-v3/include/bits/char_traits.h b/libstdc++-v3/include/bits/char_traits.h index c143d74a9ba..faccb7ba784 100644 --- a/libstdc++-v3/include/bits/char_traits.h +++ b/libstdc++-v3/include/bits/char_traits.h @@ -192,18 +192,46 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION { if (__n == 0) return __s1; -#ifdef __cpp_lib_is_constant_evaluated +#if __cpp_lib_is_constant_evaluated if (std::is_constant_evaluated()) { - if (__s1 > __s2 && __s1 < __s2 + __n) - std::copy_backward(__s2, __s2 + __n, __s1 + __n); + if (__s1 == __s2) // unlikely, but saves a lot of work + return __s1; +#if __cpp_constexpr_dynamic_alloc + // The overlap detection below fails due to PR c++/89074, + // so use a temporary buffer instead. + char_type* __tmp = new char_type[__n]; + copy(__tmp, __s2, __n); + copy(__s1, __tmp, __n); + delete[] __tmp; +#else + const auto __end = __s2 + __n - 1; + bool __overlap = false; + for (std::size_t __i = 0; __i < __n - 1; ++__i) + { + if (__s1 + __i == __end) + { + __overlap = true; + break; + } + } + if (__overlap) + { + do + { + --__n; + assign(__s1[__n], __s2[__n]); + } + while (__n > 0); + } else - std::copy(__s2, __s2 + __n, __s1); + copy(__s1, __s2, __n); +#endif return __s1; } #endif - return static_cast<_CharT*>(__builtin_memmove(__s1, __s2, - __n * sizeof(char_type))); + __builtin_memmove(__s1, __s2, __n * sizeof(char_type)); + return __s1; } template diff --git a/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc index 0d1595fe030..0fb461a2a3b 100644 --- a/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc +++ b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/constexpr_functions_c++20.cc @@ -1,5 +1,5 @@ -// { dg-options "-std=gnu++2a" } -// { dg-do compile { target c++2a } } +// { dg-options "-std=gnu++20" } +// { dg-do compile { target c++20 } } // Copyright (C) 2017-2021 Free Software Foundation, Inc. // @@ -25,9 +25,26 @@ template test_move() { using char_type = typename CT::char_type; + + // Overlapping strings char_type s1[3] = {1, 2, 3}; CT::move(s1+1, s1, 2); - return s1[0]==char_type{1} && s1[1]==char_type{1} && s1[2]==char_type{2}; + if (s1[0] != char_type{1} || s1[1] != char_type{1} || s1[2] != char_type{2}) + throw 1; + CT::move(s1, s1+1, 2); + if (s1[0] != char_type{1} || s1[1] != char_type{2} || s1[2] != char_type{2}) + throw 2; + + // Disjoint strings + char_type why_is_six_scared_of_seven[] = {4, 5, 6}; + char_type because789[] = {7, 8, 9}; + CT::move(why_is_six_scared_of_seven, because789, 3); + if (why_is_six_scared_of_seven[0] != char_type{7} + || why_is_six_scared_of_seven[1] != char_type{8} + || why_is_six_scared_of_seven[2] != char_type{9}) + throw 3; + + return true; } #ifndef __cpp_lib_constexpr_string