Optimize basic_string::operator=(const basic_string&) for SSO assignments

[mvels]

Summary

This change optimizes the operator=() assignment for short strings by direcly copying the raw data from the source into the current instance. This creates an optimized / inlined mempcy up to over 2X faster for short string assignments. With inlining enabled for operator=, performance is up to 6X faster.

Benchmarks 'as is':
name old time/op new time/op delta
BM_StringAssignStr_Empty_Opaque 6.05ns ± 2% 3.59ns ± 0% -40.67%
BM_StringAssignStr_Empty_Transparent 5.15ns ± 0% 3.08ns ± 0% -40.12%
BM_StringAssignStr_Small_Opaque 7.71ns ± 0% 3.59ns ± 0% -53.45%
BM_StringAssignStr_Small_Transparent 7.66ns ± 0% 3.09ns ± 0% -59.66%
BM_StringAssignStr_Large_Opaque 24.1ns ± 0% 24.9ns ± 0% +3.22%
BM_StringAssignStr_Large_Transparent 22.2ns ± 0% 22.8ns ± 0% +2.77%
BM_StringAssignStr_Huge_Opaque 315ns ± 6% 320ns ± 5% ~
BM_StringAssignStr_Huge_Transparent 318ns ± 5% 321ns ± 4% ~

Benchmarks with partial inlining operator=():
name old time/op new time/op delta
BM_StringAssignStr_Empty_Opaque 5.94ns ± 2% 1.95ns ± 0% -67.21%
BM_StringAssignStr_Empty_Transparent 5.14ns ± 0% 1.04ns ± 1% -79.73%
BM_StringAssignStr_Small_Opaque 7.69ns ± 0% 1.96ns ± 0% -74.48%
BM_StringAssignStr_Small_Transparent 7.65ns ± 0% 1.04ns ± 0% -86.40%
BM_StringAssignStr_Large_Opaque 24.1ns ± 0% 24.5ns ± 0% +1.61%
BM_StringAssignStr_Large_Transparent 22.2ns ± 0% 21.1ns ± 0% -4.70%
BM_StringAssignStr_Huge_Opaque 317ns ± 5% 323ns ± 4% ~
BM_StringAssignStr_Huge_Transparent 318ns ± 5% 320ns ± 5% ~

[mvels]

• Fix __assign() --> assign()

[mvels]

Scoped for pathological case to be handled outline

[mvels]

Force single branch on fast path

[EricWF]

LGTM.

[EricWF]

Committed as 58c7fa5adeb218737f10ffeaeed0d9eb21f4a2f0

libcxx/include/string

 
 template <class _CharT, class _Traits, class _Allocator>
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::operator=(const basic_string& __str)
 {
     if (this != &__str)
     {
         __copy_assign_alloc(__str);
+        const bool __str_is_long = __str.__is_long(); // Force single branch
+        if (__is_long() || __str_is_long) {
           return assign(__str.data(), __str.size());
         }
+        __r_.first().__r = __str.__r_.first().__r;
+    }
     return *this;
 }
 
 #ifndef _LIBCPP_CXX03_LANG
 
 template <class _CharT, class _Traits, class _Allocator>
 inline
 void