[libcxx-commits] [libcxx] [libc++] Refactor how we do amortized growth (PR #171117)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 updated https://github.com/llvm/llvm-project/pull/171117

>From c5ef2e8d98c5f053dde21f823edb314c41b45282 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Mon, 8 Dec 2025 13:07:30 +0100
Subject: [PATCH] [libc++] Refactor how we do amortized growth

---
 libcxx/include/string | 36 +++++++++++++++++++++---------------
 1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/libcxx/include/string b/libcxx/include/string
index 34af7efb56659..25595605bc111 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -2285,6 +2285,25 @@ private:
     return __long(__buffer, __capacity);
   }
 
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __long
+  __allocate_long_buffer_for_amortized_growth(size_type __required_size) {
+    auto __new_cap = [&] {
+      size_type __max_size = max_size();
+      if (__required_size > __max_size)
+        __throw_length_error();
+      size_type __current_cap = capacity();
+      _LIBCPP_ASSERT_INTERNAL(
+          __current_cap < __required_size, "Trying to grow string even though there is enough capacity already?");
+      if (__current_cap > __max_size / 2 - __alignment)
+        return __max_size;
+      return std::max(__required_size, 2 * __current_cap);
+    }();
+
+    __long __buffer  = __allocate_long_buffer(__alloc_, __new_cap);
+    __buffer.__size_ = __required_size;
+    return __buffer;
+  }
+
   // Replace the current buffer with __new_rep. Deallocate the old long buffer if it exists.
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __reset_internal_buffer(__rep __new_rep = __short()) {
     __annotate_delete();
@@ -2384,19 +2403,6 @@ private:
     return __guess;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type
-  __get_amortized_growth_capacity(size_type __required_capacity) {
-    size_type __max_size = max_size();
-    if (__required_capacity > __max_size)
-      __throw_length_error();
-    size_type __current_cap = capacity();
-    _LIBCPP_ASSERT_INTERNAL(
-        __current_cap < __required_capacity, "Trying to grow string even though there is enough capacity already?");
-    if (__current_cap > __max_size / 2 - __alignment)
-      return __max_size;
-    return std::max(__required_capacity, 2 * __current_cap);
-  }
-
   inline _LIBCPP_CONSTEXPR_SINCE_CXX20 void __init(const value_type* __s, size_type __sz);
   inline _LIBCPP_CONSTEXPR_SINCE_CXX20 void __init(size_type __n, value_type __c);
 
@@ -2728,7 +2734,7 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocator>::__
     size_type __n_del,
     size_type __n_add,
     const value_type* __p_new_stuff) {
-  __long __buffer = __allocate_long_buffer(__alloc_, __get_amortized_growth_capacity(__old_cap + __delta_cap));
+  __long __buffer = __allocate_long_buffer_for_amortized_growth(__old_cap + __delta_cap);
   pointer __old_p = __get_pointer();
   __annotate_delete();
   auto __guard = std::__make_scope_guard(__annotate_new_size(*this));
@@ -2761,7 +2767,7 @@ _LIBCPP_DEPRECATED_("use __grow_by_without_replace") basic_string<_CharT, _Trait
     size_type __n_copy,
     size_type __n_del,
     size_type __n_add) {
-  __long __buffer = __allocate_long_buffer(__alloc_, __get_amortized_growth_capacity(__old_cap + __delta_cap));
+  __long __buffer = __allocate_long_buffer_for_amortized_growth(__old_cap + __delta_cap);
   pointer __old_p = __get_pointer();
   if (__n_copy != 0)
     traits_type::copy(std::__to_address(__buffer.__data_), std::__to_address(__old_p), __n_copy);