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

Author: EricWF

libcxx/include/CMakeLists.txt

@@ -1044,7 +1044,7 @@ add_dependencies(cxx-headers generate-cxx-headers)
 # It's important that the arch directory be included first so that its header files
 # which interpose on the default include dir be included instead of the default ones.
 target_include_directories(cxx-headers INTERFACE ${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}
-                                                 ${LIBCXX_GENERATED_INCLUDE_DIR})
+                                                 ${CMAKE_CURRENT_LIST_DIR})
 
 if (LIBCXX_INSTALL_HEADERS)
   foreach(file ${files})

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

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) {

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;

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() {