[libc++] Introduce basic_string::__allocate_long_buffer_for_growing

[philnik777]

Introducing this utility makes the __grow_by{,_and_replace} significantly easier to understand and allows us to migrate away from these functions in the future.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

Introducing this utility makes the __grow_by{,_and_replace} significantly easier to understand and allows us to migrate away from these functions in the future.

---

Full diff: https://github.com/llvm/llvm-project/pull/162633.diff

1 Files Affected:

• (modified) libcxx/include/string (+12-12)

diff --git a/libcxx/include/string b/libcxx/include/string
index 363f27a51648c..a7b17e376675d 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -2357,6 +2357,16 @@ private:
     return __guess;
   }
 
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type __recommend_growing(size_type __required_size) {
+    size_type __max_size = max_size();
+    if (__required_size > __max_size)
+      __throw_length_error();
+    size_type __current_cap = capacity();
+    if (__current_cap > __max_size / 2 - __alignment)
+      return __max_size;
+    return std::max(__required_size, 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);
 
@@ -2701,15 +2711,10 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocator>::__
     size_type __n_del,
     size_type __n_add,
     const value_type* __p_new_stuff) {
-  size_type __ms = max_size();
-  if (__delta_cap > __ms - __old_cap)
-    __throw_length_error();
+  __long __buffer = __allocate_long_buffer(__alloc_, __recommend_growing(__old_cap + __delta_cap));
   pointer __old_p = __get_pointer();
-  size_type __cap =
-      __old_cap < __ms / 2 - __alignment ? __recommend(std::max(__old_cap + __delta_cap, 2 * __old_cap)) : __ms;
   __annotate_delete();
   auto __guard    = std::__make_scope_guard(__annotate_new_size(*this));
-  __long __buffer = __allocate_long_buffer(__alloc_, __cap);
   if (__n_copy != 0)
     traits_type::copy(std::__to_address(__buffer.__data_), std::__to_address(__old_p), __n_copy);
   if (__n_add != 0)
@@ -2739,13 +2744,8 @@ _LIBCPP_DEPRECATED_("use __grow_by_without_replace") basic_string<_CharT, _Trait
     size_type __n_copy,
     size_type __n_del,
     size_type __n_add) {
-  size_type __ms = max_size();
-  if (__delta_cap > __ms - __old_cap)
-    this->__throw_length_error();
+  __long __buffer = __allocate_long_buffer(__alloc_, __recommend_growing(__old_cap + __delta_cap));
   pointer __old_p = __get_pointer();
-  size_type __cap =
-      __old_cap < __ms / 2 - __alignment ? __recommend(std::max(__old_cap + __delta_cap, 2 * __old_cap)) : __ms;
-  __long __buffer = __allocate_long_buffer(__alloc_, __cap);
   if (__n_copy != 0)
     traits_type::copy(std::__to_address(__buffer.__data_), std::__to_address(__old_p), __n_copy);
   size_type __sec_cp_sz = __old_sz - __n_del - __n_copy;

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[ldionne]

LGTM w/ small comments and assuming no regression in the string benchmarks and SPEC.

[ldionne]

LGTM with suggestion. Please run the benchmarks to ensure there's no regression, this code is sensitive.