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

Mon Mar 24 10:40:06 PDT 2025

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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
+stable.
+
+``_LIBCPP_ABI_NO_RANDOM_DEVICE_COMPATIBILITY_LAYOUT``
+-----------------------------------------------------
+This changes the layout of ``std::random_device`` to only holds state with an implementation that gets entropy from a
+file (see ``_LIBCPP_USING_DEV_RANDOM``). When switching from this implementation to another one on a platform that has
+already shipped ``std::random_device``, one needs to retain the same object layout to remain ABI compatible. This flag
+removes these workarounds for platforms that don't care about ABI compatibility.
+
+
+``_LIBCPP_ABI_NO_COMPRESSED_PAIR_PADDING``
+------------------------------------------
+This removes artifical padding from ``_LIBCPP_COMPRESSED_PAIR`` and ``_LIBCPP_COMPRESSED_TRIPLE``. These macros are used
+inside the associative and unordered containers, ``deque``, ``forward_list``, ``future``, ``list``, ``basic_string``,
+``function``, ``shared_ptr``, ``unique_ptr``, and ``vector`` to stay ABI compatible with the legacy
+``__compressed_pair`` type. This has historically been used to reduce storage requirements in the case of empty types,
+but has been replaced by ``[[no_unique_address]]``. ``[[no_unique_address]]`` is significantly lighter in terms of
+compile time and debug information, and also improves the layout of structs further. However, to keep ABI stability, the
+additional improvements in layout had to be reverted by introducing artificial padding.
+
+``_LIBCPP_ABI_IOS_ALLOW_ARBITRARY_FILL_VALUE``
+----------------------------------------------
+``basic_ios`` uses ``WEOF`` to indicate that the fill value is uninitialized. However, on platforms where the size of
+``char_type`` is equal to or greater than the size of ``int_type`` and ``char_type`` is unsigned,
+``char_traits<char_type>::eq_int_type()`` cannot distinguish between ``WEOF`` and ``WCHAR_MAX``. This flag changes
+``basic_ios`` to instead track whether the fill value has been initialized using a separate boolean.
+
+
+Linking TUs which have been compiled against different releases of libc++
+=========================================================================
+libc++ supports linking TUs which have beeen compiled against different releases of libc++ by marking symbols with
+hidden visibility and changing the mangling of header-only functions in every release. This guarantee can be disabled
+via ``_LIBCPP_NO_ABI_TAG``, but it is not recommended to do so.
+
+
+Linking TUs which have been compiled with different flags affecting code gen
+============================================================================
+There are a lot of compiler (and library) flags which change the code generated for functions. This includes flags like
+``-O1``, which are guaranteed by the compiler to not change the observable behaviour of a correct program, as well as
+flags like ``-fexceptions``, which **do** change the observable behaviour. libc++ allows linking of TUs which have been
+compiled whith specific flags only and makes no guarantees for any of the flags not listed below.
+
+The flags allowed (in any combination) are:
+- ``-f[no-]exceptions``
+- ``-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE{_FAST,_EXTENSIVE,_DEBUG,_NONE}``
+
+Note that this does not provide any guarantees about user-defined functions, but only that the libc++ functions linked
+behave as the flags say.
+
+
+Availability of symbols in the built library (both static and shared)
+=====================================================================
+In general, libc++ does not make any guarantees about forwards-compability. That is, a TU compiled against new headers
+may not work with an older library. Vendors who require such support can leverage the availability markups. On the other
+hand, backwards compatibility is generally guaranteed.
+
+There are multiple ABI flags that change the symbols exported from the built library:
+
+``_LIBCPP_ABI_DO_NOT_EXPORT_BASIC_STRING_COMMON``
+-------------------------------------------------
+This removes ``__basic_string_common<true>::__throw_length_error()`` and
+``__basic_string_common<true>::__throw_out_of_range()``. These symbols have been used by ``basic_string`` in the past,
+but are not referenced from the headers anymore.
+
+``_LIBCPP_ABI_DO_NOT_EXPORT_VECTOR_BASE_COMMON``
+------------------------------------------------
+This removes ``__vector_base_common<true>::__throw_length_error()`` and
+``__vector_base_common<true>::__throw_out_of_range()``. These symbols have been used by ``vector`` in the past, but are
+not referenced from the headers anymore.
+
+``_LIBCPP_ABI_DO_NOT_EXPORT_TO_CHARS_BASE_10``
+----------------------------------------------
+This removes ``__itoa::__u32toa()`` and ``__iota::__u64toa``. These symbols have been used by ``to_chars`` in the past,
+but are not referenced from the headers anymore.
+
+``_LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATION``
+-------------------------------------------------------
+This replaces the symbols that are exported for ``basic_string`` to avoid exporting functions which are likely to be
+inlined as well as explicitly moving paths to the built library which are slow, improving fast-path inlining of multiple
+functions. This flag is currently unstable.
+
+
+Stability of the traits of a type
+=================================
+Whether a particular trait of a type is kept stable depends heavily on the type in question and the trait. The most
+important trait of a type to keep stable is the triviality for the purpose of calls, since that directly affects the
+function call ABI. Which types are considered non-trivial for the purpose of calls is defined in the
+`Itanium ABI <https://itanium-cxx-abi.github.io/cxx-abi/abi.html#definitions>`_.
+``is_trivially_copyable`` should also be kept stable usually, since many programs depend on this trait for their own
+layouting. This isn't rigit as the previous requirement though.
+
+There are multiple ABI flags that change traits of a struct:
+
+``_LIBCPP_ABI_ENABLE_UNIQUE_PTR_TRIVIAL_ABI``
+---------------------------------------------
+This flag adds ``[[clang::trivial_abi]]`` to ``unique_ptr``, which makes it trivial for the purpose of calls.
+
+``_LIBCPP_ABI_ENABLE_SHARED_PTR_TRIVIAL_ABI``
+---------------------------------------------
+This flag adds ``[[clang::trivial_abi]]`` to ``shared_ptr``, which makes it trivial for the purpose of calls.
+
+
+Types that public aliases reference
+===================================
+There are a lot of aliases that reference types with library internal names. For example, containers contain an
+``iterator`` alias to a type with a library internal name. These have to always reference the same type, since the
+mangling of user-defined function overloads would change otherwise. A notable exception to this are the alias templates
+to type traits. There doesn't seem to be anybody who relies on these names staying the same, so it is OK to change what
+these aliases actually reference.
+
+There are multiple ABI flags which change which type an alias references:
+
+``_LIBCPP_ABI_INCOMPLETE_TYPES_IN_DEQUE``
+-----------------------------------------
+This changes ``deque::iterator`` to avoid requring complete types for ``deque``.
+
+``_LIBCPP_ABI_FIX_UNORDERED_CONTAINER_SIZE_TYPE``
+-------------------------------------------------
+This changes the unordered container's ``size_types`` aliases.
+
+``_LIBCPP_ABI_USE_WRAP_ITER_IN_STD_ARRAY`` and ``_LIBCPP_ABI_USE_WRAP_ITER_IN_STD_STRING_VIEW``
+-----------------------------------------------------------------------------------------------
+This changes the ``iterator`` and ``const_iterator`` of ``array`` and ``string_view`` respectively to reference
+``__wrap_iter`` instead, which makes it less likely for users to depend on non-portable implementation details. This is
+especially useful because enabling bounded iterators hardening requires code not to make these assumptions.
+
+``_LIBCPP_ABI_BOUNDED_ITERATORS``, ``_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STRING``, ``_LIBCPP_ABI_BOUNDED_ITERATORS_IN_VECTOR``, and ``_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY``
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+These flags change the ``iterator`` member of various classes to reference hardened iterators instead. See the
+:ref:`hardening documentation <_hardening>` for more details.
+
+
+Meaning of values
+=================
+The meaning of specific values can usually not be changed, since programs compiled against older versions of the headers
+may check for these values. These specific values don't have to be hard-coded, but can also depend on user input.
+
+There are multiple ABI flags that change the meaning of particular values:
+
+``_LIBCPP_ABI_REGEX_CONSTANTS_NONZERO``
+---------------------------------------
+This changes the value of ``regex_constants::syntax_option-type::ECMAScript`` to be standards-conforming.
+
+``_LIBCPP_ABI_FIX_CITYHASH_IMPLEMENTATION``
+-------------------------------------------
+This flag fixes the implementation of CityHash used for ``hash<fundamental-type>``. The incorrect implementation of
+CityHash has the roblem that it drops some bits on the floor.
----------------
mordante wrote:

Since it's not obvious this is an ABI issue can you add information why it is?

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