Tested x86_64-linux. Pushed to trunk. Backports needed too. -- >8 --
The current constexpr implementation of std::char_traits<C>::move relies on being able to compare the pointer parameters, which is not allowed for unrelated pointers. We can use __builtin_constant_p to determine whether it's safe to compare the pointers directly. If not, then we know the ranges must be disjoint and so we can use char_traits<C>::copy to copy forwards from the first character to the last. If the pointers can be compared directly, then we can simplify the condition for copying backwards to just two pointer comparisons. libstdc++-v3/ChangeLog: PR libstdc++/113200 * include/bits/char_traits.h (__gnu_cxx::char_traits::move): Use __builtin_constant_p to check for unrelated pointers that cannot be compared during constant evaluation. * testsuite/21_strings/char_traits/requirements/113200.cc: New test. --- libstdc++-v3/include/bits/char_traits.h | 16 +++------------ .../char_traits/requirements/113200.cc | 20 +++++++++++++++++++ 2 files changed, 23 insertions(+), 13 deletions(-) create mode 100644 libstdc++-v3/testsuite/21_strings/char_traits/requirements/113200.cc diff --git a/libstdc++-v3/include/bits/char_traits.h b/libstdc++-v3/include/bits/char_traits.h index 018eac28d0d..3074e9bb77e 100644 --- a/libstdc++-v3/include/bits/char_traits.h +++ b/libstdc++-v3/include/bits/char_traits.h @@ -227,19 +227,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION #if __cplusplus >= 202002L if (std::__is_constant_evaluated()) { - if (__s1 == __s2) // unlikely, but saves a lot of work - return __s1; - 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) + // Use __builtin_constant_p to avoid comparing unrelated pointers. + if (__builtin_constant_p(__s2 < __s1) + && __s1 > __s2 && __s1 < (__s2 + __n)) { do { diff --git a/libstdc++-v3/testsuite/21_strings/char_traits/requirements/113200.cc b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/113200.cc new file mode 100644 index 00000000000..0fe765d53bc --- /dev/null +++ b/libstdc++-v3/testsuite/21_strings/char_traits/requirements/113200.cc @@ -0,0 +1,20 @@ +// { dg-do compile { target c++20 } } + +// PR libstdc++/113200 +// char_traits::move is not constexpr when the argument is a string literal + +#include <string_view> + +template<std::size_t N> struct S +{ + char data_[ N ]; + + constexpr S( char const* p ): data_{} + { + std::char_traits<char>::move( data_, p, N ); + } +}; + +template<std::size_t N> S( char const(&)[N] ) -> S<N>; + +constexpr S s( "test" ); -- 2.43.0