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| author | Jonathan Wakely <jwakely@redhat.com> | 2022-06-14 16:19:32 +0100 |
|---|---|---|
| committer | Jonathan Wakely <jwakely@redhat.com> | 2022-06-14 21:07:48 +0100 |
| commit | 6abe341558abec40c9c44d76e7fb4fb3978e894b (patch) | |
| tree | 70cced94b085f2c16a8eae9081e68db42407f5d1 | |
| parent | 1b65779f46f16b4fffd0591f5e58722c1e7cde8d (diff) | |
| download | gcc-6abe341558abec40c9c44d76e7fb4fb3978e894b.zip gcc-6abe341558abec40c9c44d76e7fb4fb3978e894b.tar.gz gcc-6abe341558abec40c9c44d76e7fb4fb3978e894b.tar.bz2 | |
libstdc++: Check lengths first in operator== for basic_string [PR62187]
As confirmed by LWG 2852, the calls to traits_type::compare do not need
to be obsvervable, so we can make operator== compare string lengths
first and return immediately for non-equal lengths. This avoids doing a
slow string comparison for "abc...xyz" == "abc...xy". Previously we only
did this optimization for std::char_traits<char>, but we can enable it
unconditionally thanks to LWG 2852.
For comparisons with a const char* we can call traits_type::length right
away to do the same optimization. That strlen call can be folded away
for constant arguments, making it very efficient.
For the pre-C++20 operator== and operator!= overloads we can swap the
order of the arguments to take advantage of the operator== improvements.
libstdc++-v3/ChangeLog:
PR libstdc++/62187
* include/bits/basic_string.h (operator==): Always compare
lengths before checking string contents.
[!__cpp_lib_three_way_comparison] (operator==, operator!=):
Reorder arguments.
| -rw-r--r-- | libstdc++-v3/include/bits/basic_string.h | 24 |
1 files changed, 10 insertions, 14 deletions
diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h index a34b3d9..57247e3 100644 --- a/libstdc++-v3/include/bits/basic_string.h +++ b/libstdc++-v3/include/bits/basic_string.h @@ -3627,17 +3627,10 @@ _GLIBCXX_END_NAMESPACE_CXX11 operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs, const basic_string<_CharT, _Traits, _Alloc>& __rhs) _GLIBCXX_NOEXCEPT - { return __lhs.compare(__rhs) == 0; } - - template<typename _CharT> - _GLIBCXX20_CONSTEXPR - inline - typename __gnu_cxx::__enable_if<__is_char<_CharT>::__value, bool>::__type - operator==(const basic_string<_CharT>& __lhs, - const basic_string<_CharT>& __rhs) _GLIBCXX_NOEXCEPT - { return (__lhs.size() == __rhs.size() - && !std::char_traits<_CharT>::compare(__lhs.data(), __rhs.data(), - __lhs.size())); } + { + return __lhs.size() == __rhs.size() + && !_Traits::compare(__lhs.data(), __rhs.data(), __lhs.size()); + } /** * @brief Test equivalence of string and C string. @@ -3650,7 +3643,10 @@ _GLIBCXX_END_NAMESPACE_CXX11 inline bool operator==(const basic_string<_CharT, _Traits, _Alloc>& __lhs, const _CharT* __rhs) - { return __lhs.compare(__rhs) == 0; } + { + return __lhs.size() == _Traits::length(__rhs) + && !_Traits::compare(__lhs.data(), __rhs, __lhs.size()); + } #if __cpp_lib_three_way_comparison /** @@ -3691,7 +3687,7 @@ _GLIBCXX_END_NAMESPACE_CXX11 inline bool operator==(const _CharT* __lhs, const basic_string<_CharT, _Traits, _Alloc>& __rhs) - { return __rhs.compare(__lhs) == 0; } + { return __rhs == __lhs; } // operator != /** @@ -3717,7 +3713,7 @@ _GLIBCXX_END_NAMESPACE_CXX11 inline bool operator!=(const _CharT* __lhs, const basic_string<_CharT, _Traits, _Alloc>& __rhs) - { return !(__lhs == __rhs); } + { return !(__rhs == __lhs); } /** * @brief Test difference of string and C string. |