From 703c26f03be74daf6e483380e6b23029a3851081 Mon Sep 17 00:00:00 2001
From: marshall <mclow.lists@gmail.com>
Date: Wed, 27 Nov 2019 07:13:00 -0800
Subject: [PATCH] Optimize and fix basic_string move assignment operator.
 Reviewed as https://reviews.llvm.org/D68623. Thanks to mvels for the patch.

---
 libcxx/include/string | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/libcxx/include/string b/libcxx/include/string
index c16dbedc51c0..4e0b21135a7e 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -2289,10 +2289,20 @@ basic_string<_CharT, _Traits, _Allocator>::__move_assign(basic_string& __str, tr
     _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value)
 #endif
 {
-    __clear_and_shrink();
-    __r_.first() = __str.__r_.first();
-    __move_assign_alloc(__str);
-    __str.__zero();
+  if (__is_long()) {
+    __alloc_traits::deallocate(__alloc(), __get_long_pointer(),
+                               __get_long_cap());
+#if _LIBCPP_STD_VER <= 14
+    if (!is_nothrow_move_assignable<allocator_type>::value) {
+      __set_short_size(0);
+      traits_type::assign(__get_short_pointer()[0], value_type());
+    }
+#endif
+  }
+  __move_assign_alloc(__str);
+  __r_.first() = __str.__r_.first();
+  __str.__set_short_size(0);
+  traits_type::assign(__str.__get_short_pointer()[0], value_type());
 }
 
 template <class _CharT, class _Traits, class _Allocator>