Index: libcxx/include/__config
===================================================================
--- libcxx/include/__config
+++ libcxx/include/__config
@@ -102,6 +102,9 @@
 #  define _LIBCPP_ABI_OPTIMIZED_FUNCTION
 // All the regex constants must be distinct and nonzero.
 #  define _LIBCPP_ABI_REGEX_CONSTANTS_NONZERO
+// Re-worked external template instantiations for std::string with a focus on
+// performance and fast-path inlining.
+#  define _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATIONS
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !defined(_LIBCPP_OBJECT_FORMAT_COFF)
 // Enable compiling copies of now inline methods into the dylib to support
Index: libcxx/include/__string
===================================================================
--- libcxx/include/__string
+++ libcxx/include/__string
@@ -70,17 +70,23 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+
+#ifdef _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATIONS
+#define _LIBCPP_STRING_EXTERN_TEMPLATE_LIST_ADDITIONAL(_Func, _CharType) \
+  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__init_long_external(value_type const*, size_type))
+#else
+#define _LIBCPP_STRING_EXTERN_TEMPLATE_LIST_ADDITIONAL(_Func, _CharType)
+#endif
+
 // The the extern template ABI lists are kept outside of <string> to improve the
 // readability of that header.
 #define _LIBCPP_STRING_EXTERN_TEMPLATE_LIST(_Func, _CharType) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, value_type const*, size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::rfind(value_type const*, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__init(value_type const*, size_type, size_type)) \
-  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::basic_string(basic_string const&)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, value_type const*)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::basic_string(basic_string const&, std::allocator<_CharType> const&)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_last_not_of(value_type const*, size_type, size_type) const) \
-  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::~basic_string()) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_first_not_of(value_type const*, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, size_type, value_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::operator=(value_type)) \
@@ -115,13 +121,12 @@
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find(value_type const*, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, basic_string const&, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, value_type const*, size_type) const) \
-  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::operator=(basic_string const&)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(value_type const*)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, basic_string const&, size_type, size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::iterator basic_string<_CharType>::insert(basic_string::const_iterator, value_type)) \
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::resize(size_type, value_type)) \
-  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, basic_string const&, size_type, size_type))
-
+  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, basic_string const&, size_type, size_type)) \
+  _LIBCPP_STRING_EXTERN_TEMPLATE_LIST_ADDITIONAL(_Func, _CharType)
 
 // char_traits
 
Index: libcxx/include/string
===================================================================
--- libcxx/include/string
+++ libcxx/include/string
@@ -872,7 +872,8 @@
     basic_string(initializer_list<_CharT> __il, const _Allocator& __a);
 #endif  // _LIBCPP_CXX03_LANG
 
-    inline ~basic_string();
+
+    ~basic_string();
 
     _LIBCPP_INLINE_VISIBILITY
     operator __self_view() const _NOEXCEPT { return __self_view(data(), size()); }
Index: libcxx/src/string.cpp
===================================================================
--- libcxx/src/string.cpp
+++ libcxx/src/string.cpp
@@ -18,11 +18,26 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+
 template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS __basic_string_common<true>;
 
 _LIBCPP_STRING_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, char)
 _LIBCPP_STRING_EXTERN_TEMPLATE_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, wchar_t)
 
+#define _LIBCPP_STRING_EXTERN_TEMPLATE_LEGACY_DEFINITION_LIST(_Func, _CharType) \
+  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::basic_string(basic_string const&)) \
+  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::~basic_string()) \
+  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::operator=(basic_string const&))
+
+#ifndef _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATIONS
+  // These functions are no longer declared as external in ABI v1, but still need
+  // to be defined in the dylib to maintain ABI compatibility with older libc++ versions.
+  _LIBCPP_STRING_EXTERN_TEMPLATE_LEGACY_DEFINITION_LIST(_LIBCPP_EXTERN_TEMPLATE, char)
+  _LIBCPP_STRING_EXTERN_TEMPLATE_LEGACY_DEFINITION_LIST(_LIBCPP_EXTERN_TEMPLATE, wchar_t)
+  _LIBCPP_STRING_EXTERN_TEMPLATE_LEGACY_DEFINITION_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, char)
+  _LIBCPP_STRING_EXTERN_TEMPLATE_LEGACY_DEFINITION_LIST(_LIBCPP_EXTERN_TEMPLATE_DEFINE, wchar_t)
+#endif
+
 template
     string
     operator+<char, char_traits<char>, allocator<char> >(char const*, string const&);