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Differential D72165

Optimize / partially inline basic_string assignment operator
AbandonedPublic

Authored by mvels on Jan 3 2020, 9:10 AM.

Details

Reviewers
mclow.lists
EricWF
Summary

This change adds an optimization where assigning a short string value to a short string instance is optimized into a direct memory copy.

'As is', this optimization provides a 2X to 3X+ speed up.

BM_StringAssignStr_Empty_Opaque 5.23ns ± 0% 2.27ns ± 0% -56.62% (p=0.008 n=5+5)
BM_StringAssignStr_Empty_Transparent 5.14ns ± 0% 2.27ns ± 0% -55.89% (p=0.008 n=5+5)
BM_StringAssignStr_Small_Opaque 7.69ns ± 0% 2.27ns ± 0% -70.56% (p=0.008 n=5+5)
BM_StringAssignStr_Small_Transparent 7.65ns ± 0% 2.26ns ± 0% -70.41% (p=0.008 n=5+5)
BM_StringAssignStr_Large_Opaque 24.4ns ± 0% 25.6ns ± 0% +5.11% (p=0.008 n=5+5)
BM_StringAssignStr_Large_Transparent 22.2ns ± 0% 22.9ns ± 0% +3.43% (p=0.008 n=5+5)
BM_StringAssignStr_Huge_Opaque 320ns ± 3% 320ns ± 4% ~ (p=0.841 n=5+5)
BM_StringAssignStr_Huge_Transparent 321ns ± 4% 321ns ± 4% ~ (p=1.000 n=5+5)

Subsequently changing the assignment operator to be inlined (not externally instantiated) provides a speed up of 3X to 5X.

BM_StringAssignStr_Empty_Opaque 5.25ns ± 1% 1.71ns ± 0% -67.33% (p=0.008 n=5+5)
BM_StringAssignStr_Empty_Transparent 5.13ns ± 0% 1.29ns ± 0% -74.89% (p=0.008 n=5+5)
BM_StringAssignStr_Small_Opaque 7.69ns ± 0% 1.71ns ± 0% -77.70% (p=0.008 n=5+5)
BM_StringAssignStr_Small_Transparent 7.64ns ± 0% 1.29ns ± 0% -83.10% (p=0.008 n=5+5)
BM_StringAssignStr_Large_Opaque 24.4ns ± 0% 25.3ns ± 0% +3.97% (p=0.008 n=5+5)
BM_StringAssignStr_Large_Transparent 22.2ns ± 0% 22.6ns ± 0% +2.13% (p=0.008 n=5+5)
BM_StringAssignStr_Huge_Opaque 325ns ± 5% 315ns ± 6% ~ (p=0.222 n=5+5)
BM_StringAssignStr_Huge_Transparent 325ns ± 5% 316ns ± 5% ~ (p=0.310 n=5+5)

Diff Detail

Revision Contents

PathSize
libcxx/
include/
6 lines

Diff 236076

libcxx/include/string

Show First 20 Lines • Show All 868 Lines • ▼ Show 20 Lines#ifndef _LIBCPP_CXX03_LANG
basic_string(initializer_list<_CharT> __il, const _Allocator& __a); basic_string(initializer_list<_CharT> __il, const _Allocator& __a);
#endif // _LIBCPP_CXX03_LANG #endif // _LIBCPP_CXX03_LANG
inline ~basic_string(); inline ~basic_string();
_LIBCPP_INLINE_VISIBILITY _LIBCPP_INLINE_VISIBILITY
operator __self_view() const _NOEXCEPT { return __self_view(data(), size()); } operator __self_view() const _NOEXCEPT { return __self_view(data(), size()); }
// Optimization opportunity: do not externally instantiate
basic_string& operator=(const basic_string& __str); basic_string& operator=(const basic_string& __str);
template <class _Tp, class = typename enable_if<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value, void>::type> template <class _Tp, class = typename enable_if<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value, void>::type>
basic_string& operator=(const _Tp& __t) basic_string& operator=(const _Tp& __t)
{__self_view __sv = __t; return assign(__sv);} {__self_view __sv = __t; return assign(__sv);}
#ifndef _LIBCPP_CXX03_LANG #ifndef _LIBCPP_CXX03_LANG
_LIBCPP_INLINE_VISIBILITY _LIBCPP_INLINE_VISIBILITY
▲ Show 20 Lines • Show All 1,376 Lines • ▼ Show 20 Lines
template <class _CharT, class _Traits, class _Allocator> template <class _CharT, class _Traits, class _Allocator>
basic_string<_CharT, _Traits, _Allocator>& basic_string<_CharT, _Traits, _Allocator>&
basic_string<_CharT, _Traits, _Allocator>::operator=(const basic_string& __str) basic_string<_CharT, _Traits, _Allocator>::operator=(const basic_string& __str)
{ {
if (this != &__str) if (this != &__str)
{ {
__copy_assign_alloc(__str); __copy_assign_alloc(__str);
if (__is_long() | __str.__is_long()) { // LINT: explicit binary or.
return assign(__str.data(), __str.size()); return assign(__str.data(), __str.size());
} }
__r_.first().__r = __str.__r_.first().__r;
}
return *this; return *this;
} }
#ifndef _LIBCPP_CXX03_LANG #ifndef _LIBCPP_CXX03_LANG
template <class _CharT, class _Traits, class _Allocator> template <class _CharT, class _Traits, class _Allocator>
inline inline
void void
▲ Show 20 Lines • Show All 2,103 Lines • Show Last 20 Lines