diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -496,6 +496,10 @@
 #define _LIBCPP_HAS_UNIQUE_OBJECT_REPRESENTATIONS
 #endif
 
+#if __has_builtin(__builtin_constant_p)
+#define _LIBCPP_COMPILER_HAS_BUILTIN_CONSTANT_P
+#endif
+
 #define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
 
 // Literal operators ""d and ""y are supported starting with LLVM Clang 8 and AppleClang 10.0.1
@@ -539,6 +543,10 @@
 #define _LIBCPP_HAS_UNIQUE_OBJECT_REPRESENTATIONS
 #endif
 
+#if _GNUC_VER >= 500
+#define _LIBCPP_COMPILER_HAS_BUILTIN_CONSTANT_P
+#endif
+
 #define _LIBCPP_ALWAYS_INLINE __attribute__ ((__always_inline__))
 
 #define _LIBCPP_DISABLE_EXTENSION_WARNING __extension__
@@ -1524,6 +1532,13 @@
 #  define _LIBCPP_FOPEN_CLOEXEC_MODE
 #endif
 
+
+#ifdef _LIBCPP_COMPILER_HAS_BUILTIN_CONSTANT_P
+#define _LIBCPP_BUILTIN_CONSTANT_P(x) __builtin_constant_p(x)
+#else
+#define _LIBCPP_BUILTIN_CONSTANT_P(x) false
+#endif
+
 #endif // __cplusplus
 
 #endif // _LIBCPP_CONFIG
diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1571,6 +1571,12 @@
                                size_type __n_copy,  size_type __n_del,
                                size_type __n_add, const value_type* __p_new_stuff);
 
+    _LIBCPP_INLINE_VISIBILITY
+    basic_string& __assign_inline(const value_type* __s, size_type __n);
+
+    basic_string& __assign_external(const value_type* __s, size_type __n);
+    basic_string& __assign_external(const value_type* __s);
+
     _LIBCPP_INLINE_VISIBILITY
     void __erase_to_end(size_type __pos);
 
@@ -2198,26 +2204,59 @@
 
 // assign
 
+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_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_inline(const value_type* __s, size_type __n)
+{
+  value_type* __p;
+  if (!__is_long()) {
+    __p = __get_short_pointer();
+    __set_short_size(__n);
+  } else {
+    __p = __get_long_pointer();
+    __set_long_size(__n);
+  }
+  traits_type::move(__p, __s, __n);
+  traits_type::assign(__p[__n], value_type());
+  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);
+    if (_LIBCPP_BUILTIN_CONSTANT_P(__n) && __n < __min_cap) {
+      return __assign_inline(__s, __n);
+      return *this;
+    } else {
+      return __assign_external(__s, __n);
     }
-    return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
@@ -2404,7 +2443,12 @@
 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));
+    _LIBCPP_ASSERT(__n == 0 || __s != nullptr, "string::assign received nullptr");
+    if (_LIBCPP_BUILTIN_CONSTANT_P(__s[0]) && traits_type::length(__s) < __min_cap) {
+      return __assign_inline(__s, traits_type::length(__s));
+    } else {
+      return __assign_external(__s);
+    }
 }
 
 // append