From a171402b838b9d53ca28019bfc4bcc31667e5859 Mon Sep 17 00:00:00 2001
From: Corentin Jabot <corentinjabot@gmail.com>
Date: Tue, 8 Oct 2024 15:49:29 +0200
Subject: [PATCH] try to optimize string swapping

---
 libcxx/include/__utility/swap.h | 10 +++++-----
 libcxx/include/string           | 11 ++++++++++-
 2 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/libcxx/include/__utility/swap.h b/libcxx/include/__utility/swap.h
index ed6ad3e5eb22ff..e7b23d145ea7cf 100644
--- a/libcxx/include/__utility/swap.h
+++ b/libcxx/include/__utility/swap.h
@@ -59,19 +59,19 @@ void swap_value_representations(_Tp& __a, _Tp& __b) {
   constexpr size_t __chunk = __value_size / __size;
   constexpr size_t __rem   = __value_size % __size;
 
-  // TODO: research better memswap algorithms
   char __buffer[__size];
+
   if constexpr (__chunk) {
     for (std::size_t n = 0; n < __chunk; n++, __aptr += __size, __bptr += __size) {
       __builtin_memcpy(__buffer, __aptr, __size);
-      __builtin_memmove(__aptr, __bptr, __size);
-      __builtin_memmove(__bptr, __buffer, __size);
+      __builtin_memcpy(__aptr, __bptr, __size);
+      __builtin_memcpy(__bptr, __buffer, __size);
     }
   }
   if constexpr (__rem) {
     __builtin_memcpy(__buffer, __aptr, __rem);
-    __builtin_memmove(__aptr, __bptr, __rem);
-    __builtin_memmove(__bptr, __buffer, __rem);
+    __builtin_memcpy(__aptr, __bptr, __rem);
+    __builtin_memcpy(__bptr, __buffer, __rem);
   }
 }
 #endif
diff --git a/libcxx/include/string b/libcxx/include/string
index 1fef17db545f6e..89953d839abd9a 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -750,7 +750,8 @@ struct __uninitialized_size_tag {};
 struct __init_with_sentinel_tag {};
 
 template <class _CharT, class _Traits, class _Allocator>
-class basic_string memberwise_trivially_relocatable {
+class basic_string memberwise_trivially_relocatable
+      memberwise_replaceable {
 
 private:
   using __default_allocator_type = allocator<_CharT>;
@@ -3446,6 +3447,14 @@ inline _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocat
     _NOEXCEPT_(!__alloc_traits::propagate_on_container_swap::value || __is_nothrow_swappable_v<allocator_type>)
 #endif
 {
+
+#if _LIBCPP_STD_VER >= 26
+  if constexpr(__alloc_traits::is_always_equal::value) {
+    std::swap_value_representations(*this, __str);
+    return;
+  }
+#endif
+
   _LIBCPP_ASSERT_COMPATIBLE_ALLOCATOR(
       __alloc_traits::propagate_on_container_swap::value || __alloc_traits::is_always_equal::value ||
           __alloc() == __str.__alloc(),