diff --git a/libcxx/include/__string b/libcxx/include/__string
--- a/libcxx/include/__string
+++ b/libcxx/include/__string
@@ -337,8 +337,20 @@
 
     static _LIBCPP_CONSTEXPR_AFTER_CXX14
     int compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
-    static inline size_t _LIBCPP_CONSTEXPR_AFTER_CXX14
-    length(const char_type* __s)  _NOEXCEPT {return __builtin_strlen(__s);}
+
+    static inline size_t _LIBCPP_CONSTEXPR_AFTER_CXX14 length(const char_type* __s)  _NOEXCEPT {
+#ifdef _LIBCPP_COMPILER_GCC
+    if (__libcpp_is_constant_evaluated()) {
+      size_t __i = 0;
+      for (; __s[i] != '\0'; ++__i)
+        ;
+      return __i;
+    }
+#endif
+
+      return __builtin_strlen(__s);
+    }
+
     static _LIBCPP_CONSTEXPR_AFTER_CXX14
     const char_type* find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT;
     static inline _LIBCPP_CONSTEXPR_AFTER_CXX17