[libcxx-commits] [libcxx] [libc++] Document our ABI guarantees and what ABI flags exist to modify these guarantees (PR #132615)

[Mark de Wever via libcxx-commits]

================
@@ -0,0 +1,199 @@
+=======================
+libc++'s ABI Guarantees
+=======================
+
+libc++ provides multiple types ABI guarantees. These include stability of the layout of structs, the linking of TUs
+built against different versions and configurations of the library, and more. This document describes what guarantees
+libc++ provides in these different fields as well as what options exist for users and vendors to affect these
+guarantees.
+
+Note that all of the guarantees listed below come with an asterisk that there may be circumstances where we deem it
+worth it to break that guarantee. These breaks are communicated to vendors by CCing #libcxx-vendors on GitHub. If you
+are a vendor, please ask to be added to that group to be notified about changes that potentially affect you.
+
+
+Stability of the Layout of Structs
+==================================
+
+The layout of any struct that is observable by the user is kept stable across versions of the library and any options
+users are allowed to change. There are a lot of structs that have internal names, but are none the less observable by
+users; for example through public aliases to these types or because they affect the layout of other types.
+
+There are multiple ABI flags which affect the layout of certain structs:
+
+``_LIBCPP_ABI_ALTERNATE_STRING_LAYOUT``
+---------------------------------------
+This changes the internal layout of ``basic_string`` to move the section that is used for the internal buffer to the
+front, making it eight byte aligned instead of being unaligned, improving the performance of some operations
+significantly.
+
+``_LIBCPP_ABI_NO_ITERATOR_BASES``
+---------------------------------
+This removes the ``std::iterator`` base class from ``back_insert_iterator``, ``front_insert_iterator``,
----------------
mordante wrote:

You use `std::foo` and `foo`, I'd like to see a consistent way. Look at the document, dropping the `std::` part seems the easiest solution.

https://github.com/llvm/llvm-project/pull/132615