diff --git a/libcxx/include/__string b/libcxx/include/__string
--- a/libcxx/include/__string
+++ b/libcxx/include/__string
@@ -152,7 +152,8 @@
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, value_type const*, size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_first_of(value_type const*, size_type, size_type) const) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, size_type, value_type)) \
-  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(value_type const*, size_type)) \
+  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::__assign_external(value_type const*, size_type)) \
+  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::__assign_external(value_type const*)) \
   _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::reserve(size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(value_type const*, size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(basic_string const&, size_type, size_type)) \
@@ -176,7 +177,6 @@
   _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(value_type const*) const) \
   _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, value_type const*) const) \
   _Func(_LIBCPP_FUNC_VIS _CharType& basic_string<_CharType>::at(size_type)) \
-  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(value_type const*)) \
   _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) \
@@ -184,6 +184,8 @@
   _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 _CharType* basic_string<_CharType>::__resize<false>(size_type)) \
+  _Func(_LIBCPP_FUNC_VIS _CharType* basic_string<_CharType>::__resize<true>(size_type)) \
   _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, basic_string const&, size_type, size_type))
 
 
diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1541,6 +1541,12 @@
         if (__guess == __min_cap) ++__guess;
         return __guess;
         }
+    static _LIBCPP_INLINE_VISIBILITY size_type
+        __recommend_cap(size_type __cap) _NOEXCEPT {
+      return __align_it < sizeof(value_type) < __alignment
+          ? __alignment / sizeof(value_type)
+          : 1 > (__cap);
+    }
 
     inline
     void __init(const value_type* __s, size_type __sz, size_type __reserve);
@@ -1662,6 +1668,35 @@
         _NOEXCEPT
         {}
 
+    basic_string& __assign_external(const value_type* __s);
+    basic_string& __assign_external(const value_type* __s, size_type __n);
+
+    template <bool __is_short>
+    value_type* __resize(size_type __n);
+
+    inline value_type* __resize(size_type __n) {
+      return __is_long() ? __resize<false>(__n) : __resize<true>(__n);
+    }
+
+    inline value_type* __resize_small(size_type __n) _NOEXCEPT {
+      pointer __p = __is_long()
+                        ? (__set_long_size(__n), __get_long_pointer())
+                        : (__set_short_size(__n), __get_short_pointer());
+      traits_type::assign(__p[__n], value_type());
+      return  _VSTD::__to_address(__p);
+    }
+
+    inline basic_string& __assign_small_length(const value_type* __s,
+                                               size_type __n) {
+      value_type* p = (__n < __min_cap) ? __resize_small(__n) : __resize(__n);
+      if (__builtin_constant_p(*__s)) {
+        traits_type::copy(p, __s, __n);
+      } else {
+        traits_type::move(p, __s, __n);
+      }
+      return *this;
+    }
+
     _LIBCPP_INLINE_VISIBILITY void __invalidate_all_iterators();
     _LIBCPP_INLINE_VISIBILITY void __invalidate_iterators_past(size_type);
 
@@ -2221,6 +2256,38 @@
     traits_type::assign(__p[__old_sz], value_type());
 }
 
+template <class _CharT, class _Traits, class _Allocator>
+template <bool __is_short>
+_CharT* basic_string<_CharT, _Traits, _Allocator>::__resize(size_type __n) {
+  size_type __old_cap = __is_short ? __min_cap : __get_long_cap();
+  pointer __old_p = __is_short ? __get_short_pointer() : __get_long_pointer();
+  if (__n < __old_cap) {
+    __is_short ? __set_short_size(__n) : __set_long_size(__n);
+    return _VSTD::__to_address(__old_p);
+  }
+
+  const size_type __ms = max_size();
+  if (__n > __ms) this->__throw_length_error();
+
+  size_type __new_cap;
+  if (__is_short) {
+    __new_cap = __recommend_cap(_VSTD::max<size_type>(__n + 1, 32));
+  } else {
+    __new_cap = (__old_cap < __ms / 2 - __alignment)
+                    ? __recommend_cap(_VSTD::max(__n + 1, 2 * __old_cap))
+                    : __ms;
+  }
+  pointer __p = __alloc_traits::allocate(__alloc(), __new_cap);
+  __invalidate_all_iterators();
+  __set_long_pointer(__p);
+  __is_short ? __set_short_size(__n) : __set_long_size(__n);
+  __set_long_cap(__new_cap);
+  if (!__is_short) {
+    __alloc_traits::deallocate(__alloc(), __old_p, __old_cap);
+  }
+  return __p;
+}
+
 template <class _CharT, class _Traits, class _Allocator>
 void
 basic_string<_CharT, _Traits, _Allocator>::__grow_by(size_type __old_cap, size_type __delta_cap, size_type __old_sz,
@@ -2270,26 +2337,32 @@
   return *this;
 }
 
+template <class _CharT, class _Traits, class _Allocator>
+basic_string<_CharT, _Traits, _Allocator>&
+basic_string<_CharT, _Traits, _Allocator>::__assign_external(
+    const value_type* __s, size_type __n) {
+  size_type __cap = capacity();
+  if (__cap >= __n) {
+    value_type* __p = _VSTD::__to_address(__get_pointer());
+    traits_type::move(__p, __s, __n);
+    traits_type::assign(__p[__n], value_type());
+    __set_size(__n);
+    __invalidate_iterators_past(__n);
+  } else {
+    size_type __sz = size();
+    __grow_by_and_replace(__cap, __n - __cap, __sz, 0, __sz, __n, __s);
+  }
+  return *this;
+}
+
 template <class _CharT, class _Traits, class _Allocator>
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::assign(const value_type* __s, size_type __n)
 {
     _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::assign received nullptr");
-    size_type __cap = capacity();
-    if (__cap >= __n)
-    {
-        value_type* __p = _VSTD::__to_address(__get_pointer());
-        traits_type::move(__p, __s, __n);
-        traits_type::assign(__p[__n], value_type());
-        __set_size(__n);
-        __invalidate_iterators_past(__n);
-    }
-    else
-    {
-        size_type __sz = size();
-        __grow_by_and_replace(__cap, __n - __cap, __sz, 0, __sz, __n, __s);
-    }
-    return *this;
+    return (__builtin_constant_p(__n) && __n <= 128)
+               ? __assign_small_length(__s, __n)
+               : __assign_external(__s, __n);
 }
 
 template <class _CharT, class _Traits, class _Allocator>
@@ -2474,14 +2547,21 @@
 }
 
 
+template <class _CharT, class _Traits, class _Allocator>
+basic_string<_CharT, _Traits, _Allocator>&
+basic_string<_CharT, _Traits, _Allocator>::__assign_external(const value_type* __s) {
+  return __assign_external(__s, traits_type::length(__s));
+}
+
 template <class _CharT, class _Traits, class _Allocator>
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::assign(const value_type* __s)
 {
     _LIBCPP_ASSERT(__s != nullptr, "string::assign received nullptr");
-    return assign(__s, traits_type::length(__s));
+    return (__builtin_constant_p(*__s) && traits_type::length(__s) <= 128)
+               ? __assign_small_length(__s, traits_type::length(__s))
+               : __assign_external(__s);
 }
-
 // append
 
 template <class _CharT, class _Traits, class _Allocator>