* [committed] libstdc++: Check lengths first in operator== for basic_string [PR62187]
@ 2022-06-14 20:20 Jonathan Wakely
0 siblings, 0 replies; only message in thread
From: Jonathan Wakely @ 2022-06-14 20:20 UTC (permalink / raw)
To: libstdc++, gcc-patches
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
^ permalink raw reply [flat|nested] only message in thread
only message in thread, other threads:[~2022-06-14 20:20 UTC | newest]
Thread overview: (only message) (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-06-14 20:20 [committed] libstdc++: Check lengths first in operator== for basic_string [PR62187] Jonathan Wakely
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).