diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -102,6 +102,10 @@
 #  define _LIBCPP_ABI_OPTIMIZED_FUNCTION
 // All the regex constants must be distinct and nonzero.
 #  define _LIBCPP_ABI_REGEX_CONSTANTS_NONZERO
+// Inline assign(s, [, n]) code, performing inline assignment for strings
+// which length is known at compile time and smaller than __min_cap, resulting
+// in up to 6X speed ups for (asciiz) string assignments.
+#  define _LIBCPP_ABI_PARTIALLY_INLINE_ASSIGN
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !defined(_LIBCPP_OBJECT_FORMAT_COFF)
 // Enable compiling copies of now inline methods into the dylib to support
@@ -488,6 +492,10 @@
 #define _LIBCPP_COMPILER_HAS_BUILTIN_LAUNDER
 #endif
 
+#if defined(__GNUC__) || __has_builtin(__builtin_constant_p)
+#define _LIBCPP_COMPILER_HAS_BUILTIN_CONSTANT_P
+#endif
+
 #if !__is_identifier(__has_unique_object_representations)
 #define _LIBCPP_HAS_UNIQUE_OBJECT_REPRESENTATIONS
 #endif
diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -796,6 +796,10 @@
         _NOEXCEPT;
 #endif
 
+    // Optimization opportunity: do not externally instantiate the copy
+    // constructor, which inlines short string initialization. Long string
+    // initialization is delegated to the (external) __init_long()method,
+    // which results in a 3X-4X speed up for SSO initialization.
     basic_string(const basic_string& __str);
     basic_string(const basic_string& __str, const allocator_type& __a);
 
@@ -874,6 +878,7 @@
     _LIBCPP_INLINE_VISIBILITY
     operator __self_view() const _NOEXCEPT { return __self_view(data(), size()); }
 
+    // Optimization opportunity: do not externally instantiate
     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>
@@ -1084,8 +1089,17 @@
             basic_string&
         >::type
                   assign(const _Tp & __t, size_type __pos, size_type __n=npos);
+
+#ifdef _LIBCPP_ABI_PARTIALLY_INLINE_ASSIGN
+    _LIBCPP_INLINE_VISIBILITY
+    basic_string& assign(const value_type* __s, size_type __n);
+    _LIBCPP_INLINE_VISIBILITY
+    basic_string& assign(const value_type* __s);
+#else
     basic_string& assign(const value_type* __s, size_type __n);
     basic_string& assign(const value_type* __s);
+#endif
+
     basic_string& assign(size_type __n, value_type __c);
     template<class _InputIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
@@ -1543,6 +1557,8 @@
     inline
     void __init(size_type __n, value_type __c);
 
+    void __init_long(const basic_string& __str);
+
     template <class _InputIterator>
     inline
     typename enable_if
@@ -1604,6 +1620,21 @@
     void __copy_assign_alloc(const basic_string&, false_type) _NOEXCEPT
         {}
 
+#ifdef _LIBCPP_ABI_PARTIALLY_INLINE_ASSIGN
+    basic_string& __assign(const value_type* __s, size_type __n);
+    basic_string& __assign(const value_type* __s);
+#else
+    _LIBCPP_INLINE_VISIBILITY
+    basic_string& __assign(const value_type* __s, size_type __n);
+    _LIBCPP_INLINE_VISIBILITY
+    basic_string& __assign(const value_type* __s);
+#endif
+
+#ifdef _LIBCPP_COMPILER_HAS_BUILTIN_CONSTANT_P
+    _LIBCPP_INLINE_VISIBILITY
+    basic_string& __assign_in_place(const value_type* __s, size_type __n);
+#endif
+
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
     void __move_assign(basic_string& __str, false_type)
@@ -1794,6 +1825,20 @@
     traits_type::assign(__p[__sz], value_type());
 }
 
+template <class _CharT, class _Traits, class _Allocator>
+void basic_string<_CharT, _Traits, _Allocator>::__init_long(
+    const basic_string& __str) {
+  const value_type* __s = _VSTD::__to_address(__str.__get_long_pointer());
+  size_type __sz = __str.__get_long_size();
+  size_type __cap = __recommend(__sz);
+  pointer __p = __alloc_traits::allocate(__alloc(), __cap + 1);
+  __set_long_pointer(__p);
+  __set_long_cap(__cap + 1);
+  __set_long_size(__sz);
+  traits_type::copy(_VSTD::__to_address(__p), __s, __sz);
+  traits_type::assign(__p[__sz], value_type());
+}
+
 template <class _CharT, class _Traits, class _Allocator>
 template <class>
 basic_string<_CharT, _Traits, _Allocator>::basic_string(const _CharT* __s, const _Allocator& __a)
@@ -1837,7 +1882,7 @@
     if (!__str.__is_long())
         __r_.first().__r = __str.__r_.first().__r;
     else
-        __init(_VSTD::__to_address(__str.__get_long_pointer()), __str.__get_long_size());
+        __init_long(__str);
 #if _LIBCPP_DEBUG_LEVEL >= 2
     __get_db()->__insert_c(this);
 #endif
@@ -1851,7 +1896,7 @@
     if (!__str.__is_long())
         __r_.first().__r = __str.__r_.first().__r;
     else
-        __init(_VSTD::__to_address(__str.__get_long_pointer()), __str.__get_long_size());
+        __init_long(__str);
 #if _LIBCPP_DEBUG_LEVEL >= 2
     __get_db()->__insert_c(this);
 #endif
@@ -2197,11 +2242,29 @@
 
 // assign
 
+template <class _CharT, class _Traits, class _Allocator>
+inline
+basic_string<_CharT, _Traits, _Allocator>&
+basic_string<_CharT, _Traits, _Allocator>::__assign_in_place(
+    const value_type* __s, size_type __n) {
+  pointer __p;
+  if (__is_long()) {
+    __p = __get_long_pointer();
+    __set_long_size(__n);
+  } else {
+    __p = __get_short_pointer();
+    __set_short_size(__n);
+  }
+  traits_type::copy(_VSTD::__to_address(__p), __s, __n);
+  traits_type::assign(__p[__n], value_type());
+  __invalidate_iterators_past(__n);
+  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)
+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)
     {
@@ -2219,6 +2282,25 @@
     return *this;
 }
 
+template <class _CharT, class _Traits, class _Allocator>
+basic_string<_CharT, _Traits, _Allocator>&
+basic_string<_CharT, _Traits, _Allocator>::__assign(const value_type* __s)
+{
+    return __assign(__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, size_type __n) {
+    _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::assign received nullptr");
+#ifdef _LIBCPP_COMPILER_HAS_BUILTIN_CONSTANT_P
+    if (__builtin_constant_p(__n) && __n < __min_cap) {
+        return __assign_in_place(__s, __n);
+    }
+#endif
+    return __assign(__s, __n);
+}
+
 template <class _CharT, class _Traits, class _Allocator>
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::assign(size_type __n, value_type __c)
@@ -2266,7 +2348,10 @@
     if (this != &__str)
     {
         __copy_assign_alloc(__str);
-        return assign(__str.data(), __str.size());
+        if (__is_long() | __str.__is_long()) {  // LINT: explicit binary or.
+          return assign(__str.data(), __str.size());
+        }
+        __r_.first().__r = __str.__r_.first().__r;
     }
     return *this;
 }
@@ -2398,7 +2483,12 @@
 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));
+#ifdef _LIBCPP_COMPILER_HAS_BUILTIN_CONSTANT_P
+    if (__builtin_constant_p(__s[0]) && traits_type::length(__s) < __min_cap) {
+      return __assign_in_place(__s, traits_type::length(__s));
+    }
+#endif
+    return __assign(__s);
 }
 
 // append