[libcxx-commits] [libcxx] [libc++][NFC] Use __constexpr_memmove instead of copy_n in <__string/char_traits.h> (PR #85920)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 created https://github.com/llvm/llvm-project/pull/85920

`copy_n` has been used to allow constant evaluation of `char_traits`. We now have `__constexpr_memmove`, which `copy_n` just forwards to. We can call `__constexpr_memmove` directly, avoiding a bunch of instantiations. This reduces the time it takes to include `<string>` from 321ms to 285ms.


>From 5650f427a87bdd6e88e866bbae4b64d1fbf5899a Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Wed, 20 Mar 2024 12:36:31 +0100
Subject: [PATCH] [libc++][NFC] Use __constexpr_memmove instead of copy_n in
 <__string/char_traits.h>

---
 libcxx/include/__string/char_traits.h | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/libcxx/include/__string/char_traits.h b/libcxx/include/__string/char_traits.h
index 5880d3a22db2e7..47ed1057caaab1 100644
--- a/libcxx/include/__string/char_traits.h
+++ b/libcxx/include/__string/char_traits.h
@@ -9,7 +9,6 @@
 #ifndef _LIBCPP___STRING_CHAR_TRAITS_H
 #define _LIBCPP___STRING_CHAR_TRAITS_H
 
-#include <__algorithm/copy_n.h>
 #include <__algorithm/fill_n.h>
 #include <__algorithm/find_end.h>
 #include <__algorithm/find_first_of.h>
@@ -144,7 +143,7 @@ struct _LIBCPP_TEMPLATE_VIS char_traits<char> {
   copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
     _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(!std::__is_pointer_in_range(__s1, __s1 + __n, __s2),
                                           "char_traits::copy: source and destination ranges overlap");
-    std::copy_n(__s2, __n, __s1);
+    std::__constexpr_memmove(__s1, __s2, __element_count(__n));
     return __s1;
   }
 
@@ -221,7 +220,7 @@ struct _LIBCPP_TEMPLATE_VIS char_traits<wchar_t> {
   copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
     _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(!std::__is_pointer_in_range(__s1, __s1 + __n, __s2),
                                           "char_traits::copy: source and destination ranges overlap");
-    std::copy_n(__s2, __n, __s1);
+    std::__constexpr_memmove(__s1, __s2, __element_count(__n));
     return __s1;
   }
 
@@ -287,7 +286,7 @@ struct _LIBCPP_TEMPLATE_VIS char_traits<char8_t> {
   copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
     _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(!std::__is_pointer_in_range(__s1, __s1 + __n, __s2),
                                           "char_traits::copy: source and destination ranges overlap");
-    std::copy_n(__s2, __n, __s1);
+    std::__constexpr_memmove(__s1, __s2, __element_count(__n));
     return __s1;
   }
 
@@ -366,7 +365,7 @@ struct _LIBCPP_TEMPLATE_VIS char_traits<char16_t> {
   copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
     _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(!std::__is_pointer_in_range(__s1, __s1 + __n, __s2),
                                           "char_traits::copy: source and destination ranges overlap");
-    std::copy_n(__s2, __n, __s1);
+    std::__constexpr_memmove(__s1, __s2, __element_count(__n));
     return __s1;
   }
 
@@ -454,7 +453,7 @@ struct _LIBCPP_TEMPLATE_VIS char_traits<char32_t> {
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static char_type*
   copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
-    std::copy_n(__s2, __n, __s1);
+    std::__constexpr_memmove(__s1, __s2, __element_count(__n));
     return __s1;
   }