[libcxx-commits] [libcxx] [libc++] Base string's alignment on __STDCPP_DEFAULT_NEW_ALIGNMENT__ (PR #171785)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 updated https://github.com/llvm/llvm-project/pull/171785

>From 4e583cf0a33d9488cda67b7d95e49cba6bb4df8e Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Thu, 11 Dec 2025 10:14:52 +0100
Subject: [PATCH] [libc++] Base string's alignment on
 __STDCPP_DEFAULT_NEW_ALIGNMENT__

---
 libcxx/include/string                                  | 10 +++++++++-
 .../string.capacity/allocation_size.pass.cpp           |  7 +++++--
 .../basic.string/string.capacity/max_size.pass.cpp     |  7 +++++--
 3 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/libcxx/include/string b/libcxx/include/string
index 1ced528135b97..7b71058427ba4 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -2351,7 +2351,15 @@ private:
   static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type __align_it(size_type __s) _NOEXCEPT {
     return (__s + (__a - 1)) & ~(__a - 1);
   }
-  enum { __alignment = 8 };
+
+#  ifdef __STDCPP_DEFAULT_NEW_ALIGNMENT__
+  static inline const size_t
+      __alignment = __is_std_allocator_v<allocator_type> && __STDCPP_DEFAULT_NEW_ALIGNMENT__ > sizeof(void*)
+                      ? __STDCPP_DEFAULT_NEW_ALIGNMENT__
+                      : sizeof(void*);
+#  else
+  static inline const size_t __alignment = sizeof(void*);
+#  endif
 
   // This makes sure that we're using a capacity with some extra alignment, since allocators almost always over-align
   // the allocations anyways, improving memory usage. More importantly, this ensures that the lowest bit is never set
diff --git a/libcxx/test/libcxx/strings/basic.string/string.capacity/allocation_size.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.capacity/allocation_size.pass.cpp
index 77da29225957b..367a58897cec0 100644
--- a/libcxx/test/libcxx/strings/basic.string/string.capacity/allocation_size.pass.cpp
+++ b/libcxx/test/libcxx/strings/basic.string/string.capacity/allocation_size.pass.cpp
@@ -15,8 +15,11 @@
 
 #include "test_macros.h"
 
-// alignment of the string heap buffer is hardcoded to 8
-const std::size_t alignment = 8;
+#ifdef __STDCPP_DEFAULT_NEW_ALIGNMENT__
+static const std::size_t alignment = __STDCPP_DEFAULT_NEW_ALIGNMENT__;
+#else
+static const std::size_t alginment = 8;
+#endif
 
 int main(int, char**) {
   std::string input_string;
diff --git a/libcxx/test/libcxx/strings/basic.string/string.capacity/max_size.pass.cpp b/libcxx/test/libcxx/strings/basic.string/string.capacity/max_size.pass.cpp
index 6bfcb5d4bfcd8..ea4520031475b 100644
--- a/libcxx/test/libcxx/strings/basic.string/string.capacity/max_size.pass.cpp
+++ b/libcxx/test/libcxx/strings/basic.string/string.capacity/max_size.pass.cpp
@@ -17,8 +17,11 @@
 
 #include "test_macros.h"
 
-// alignment of the string heap buffer is hardcoded to 8
-static const std::size_t alignment = 8;
+#ifdef __STDCPP_DEFAULT_NEW_ALIGNMENT__
+static const std::size_t alignment = __STDCPP_DEFAULT_NEW_ALIGNMENT__;
+#else
+static const std::size_t alginment = 8;
+#endif
 
 template <class = int>
 TEST_CONSTEXPR_CXX20 void full_size() {