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

[Louis Dionne via libcxx-commits]

================
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+=======================
+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``,
+``insert_iterator``, ``istream_iterator``, ``ostream_iterator``, ``ostreambuf_itreator``, ``reverse_iterator``, and
+``raw_storage_iterator``. This doesn't directly affect the layout of these types in most cases, but may result in more
+padding being used when they are used in combination, for example ``reverse_iterator<reverse_iterator<T>>``.
+
+- ``_LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION``
+-------------------------------------------------
+This changes the index type used inside ``std::variant`` to the smallest required type to reduce the datasize of
+variants in most cases.
+
+``_LIBCPP_ABI_OPTIMIZED_FUNCTION``
+----------------------------------
+This significantly restructures how `std::function` is written to provide better performance, but is currently not ABI
----------------
ldionne wrote:

```suggestion
This significantly restructures how ``std::function`` is written to provide better performance, but is currently not ABI
```

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