From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 2181) id 4E12F385AC2B; Thu, 18 Nov 2021 16:14:20 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 4E12F385AC2B 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 r12-5376] libstdc++: Fix std::char_traits::move for constexpr X-Act-Checkin: gcc X-Git-Author: Jonathan Wakely X-Git-Refname: refs/heads/master X-Git-Oldrev: 16fab6d691b1410abdf794230246ab9753d183af X-Git-Newrev: ca243ada71656651a8753e88164a1f0f019be1c3 Message-Id: <20211118161420.4E12F385AC2B@sourceware.org> Date: Thu, 18 Nov 2021 16:14:20 +0000 (GMT) X-BeenThere: gcc-cvs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-cvs mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 18 Nov 2021 16:14:20 -0000 https://gcc.gnu.org/g:ca243ada71656651a8753e88164a1f0f019be1c3 commit r12-5376-gca243ada71656651a8753e88164a1f0f019be1c3 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. 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 f6f8851c22d..3c9f4ad9420 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 0577cb1c0bc..56400bb18c9 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