From: Jonathan Wakely <jwakely@redhat.com>
To: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org
Subject: [committed] libstdc++: Check lengths first in operator== for basic_string [PR62187]
Date: Tue, 14 Jun 2022 21:20:00 +0100 [thread overview]
Message-ID: <20220614202000.1030288-1-jwakely@redhat.com> (raw)
Tested powerpc64le-linux, pushed to trunk.
-- >8 --
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.
---
libstdc++-v3/include/bits/basic_string.h | 24 ++++++++++--------------
1 file changed, 10 insertions(+), 14 deletions(-)
diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index a34b3d9ed28..57247e306dc 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.
--
2.34.3
reply other threads:[~2022-06-14 20:20 UTC|newest]
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