[libcxx-commits] [libcxx] [libc++] Optimize vector assign for trivially copyable types (PR #233863)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 updated https://github.com/llvm/llvm-project/pull/233863

>From 971fa28c04f2233d0f5c4546767a9461ae64fa15 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Tue, 20 Oct 2026 13:16:15 +0200
Subject: [PATCH] [libc++] Optimize vector assign for trivially copyable types

---
 libcxx/include/__vector/vector.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/libcxx/include/__vector/vector.h b/libcxx/include/__vector/vector.h
index 37a25f20e65cd4..6dfea9b2046485 100644
--- a/libcxx/include/__vector/vector.h
+++ b/libcxx/include/__vector/vector.h
@@ -48,6 +48,7 @@
 #include <__ranges/container_compatible_range.h>
 #include <__ranges/from_range.h>
 #include <__split_buffer>
+#include <__string/constexpr_c_functions.h>
 #include <__type_traits/conditional.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_allocator.h>
@@ -59,6 +60,7 @@
 #include <__type_traits/is_relocatable.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_swappable.h>
+#include <__type_traits/is_trivially_copyable.h>
 #include <__type_traits/remove_const_ref.h>
 #include <__type_traits/type_identity.h>
 #include <__utility/assume.h>
@@ -968,6 +970,26 @@ template <class _AlgPolicy, class _Iterator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void
 vector<_Tp, _Allocator>::__assign_with_size(_Iterator __first, difference_type __n) {
   size_type __new_size = static_cast<size_type>(__n);
+
+#ifndef _LIBCPP_CXX03_LANG
+  if constexpr (is_same<value_type, __policy_value_type<_AlgPolicy, _Iterator>>::value &&
+                __allocator_has_trivial_move_construct_v<allocator_type, value_type> &&
+                __allocator_has_trivial_destroy_v<allocator_type, value_type> &&
+                is_trivially_copyable<value_type>::value && __libcpp_is_contiguous_iterator<_Iterator>::value) {
+    if (!__libcpp_is_constant_evaluated()) {
+      if (__new_size > capacity()) {
+        __vdeallocate();
+        __vallocate(__recommend(__new_size));
+      }
+
+      std::__constexpr_memmove(
+          std::__to_address(__layout_.__begin_ptr()), std::__to_address(__first), __element_count(__new_size));
+      __layout_.__set_bound_using_pointer(__layout_.__begin_ptr() + __new_size);
+      return;
+    }
+  }
+#endif
+
   if (__new_size <= capacity()) {
     auto const __size = size();
     if (__new_size > __size) {