From 7523a5612d2fbd5c8b1bead6219d2b5c25946ca1 Mon Sep 17 00:00:00 2001
From: Eric Fiselier <eric@efcs.ca>
Date: Mon, 22 Jan 2024 16:37:40 -0600
Subject: [PATCH] Unconditionally change std::string's alignment to 8.

    Save memory by providing the allocator more freedom to allocate the most
    efficient size class by dropping the alignment requirements for std::string's
    pointer from 16 to 8. This changes the output of std::string::max_size,
    which makes it ABI breaking.

    That said, the discussion concluded that we don't care about this ABI
    break and would like this change enabled universally
---
 libcxx/include/__config                                |  5 -----
 libcxx/include/string                                  |  7 +------
 .../string.capacity/allocation_size.pass.cpp           | 10 ++--------
 .../basic.string/string.capacity/max_size.pass.cpp     | 10 ++--------
 4 files changed, 5 insertions(+), 27 deletions(-)

diff --git a/libcxx/include/__config b/libcxx/include/__config
index 9a64cdb489119d..6fd44a3b915f5d 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -174,11 +174,6 @@
 // The implementation moved to the header, but we still export the symbols from
 // the dylib for backwards compatibility.
 #    define _LIBCPP_ABI_DO_NOT_EXPORT_TO_CHARS_BASE_10
-// Save memory by providing the allocator more freedom to allocate the most
-// efficient size class by dropping the alignment requirements for std::string's
-// pointer from 16 to 8. This changes the output of std::string::max_size,
-// which makes it ABI breaking
-#    define _LIBCPP_ABI_STRING_8_BYTE_ALIGNMENT
 #  elif _LIBCPP_ABI_VERSION == 1
 #    if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
 // Enable compiling copies of now inline methods into the dylib to support
diff --git a/libcxx/include/string b/libcxx/include/string
index e97139206d4fa7..ba169c3dbfc9e6 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1937,12 +1937,7 @@ private:
     return (__s + (__a - 1)) & ~(__a - 1);
   }
   enum {
-    __alignment =
-#ifdef _LIBCPP_ABI_STRING_8_BYTE_ALIGNMENT
-        8
-#else
-        16
-#endif
+    __alignment = 8
   };
   static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type __recommend(size_type __s) _NOEXCEPT {
     if (__s < __min_cap) {
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 c7df56c815a805..6791ada40894c7 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
@@ -17,14 +17,8 @@
 
 #include "test_macros.h"
 
-// alignment of the string heap buffer is hardcoded to either 16 or 8
-
-const std::size_t alignment =
-#ifdef _LIBCPP_ABI_STRING_8_BYTE_ALIGNMENT
-    8;
-#else
-    16;
-#endif
+// alignment of the string heap buffer is hardcoded to 8
+const std::size_t alignment = 8;
 
 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 a3cb79522f2e1e..726570beb6d1ae 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
@@ -17,14 +17,8 @@
 
 #include "test_macros.h"
 
-// alignment of the string heap buffer is hardcoded to 16
-
-static const std::size_t alignment =
-#ifdef _LIBCPP_ABI_STRING_8_BYTE_ALIGNMENT
-    8;
-#else
-    16;
-#endif
+// alignment of the string heap buffer is hardcoded to 8
+static const std::size_t alignment = 8;
 
 template <class = int>
 TEST_CONSTEXPR_CXX20 void full_size() {