llvm-project/clang/docs/ReleaseNotes.rst

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Clang |release| |ReleaseNotesTitle|
===========================================

.. contents::
   :local:
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Written by the `LLVM Team <https://llvm.org/>`_

.. only:: PreRelease

  .. warning::
     These are in-progress notes for the upcoming Clang |version| release.
     Release notes for previous releases can be found on
     `the Releases Page <https://llvm.org/releases/>`_.

Introduction
============

This document contains the release notes for the Clang C/C++/Objective-C
frontend, part of the LLVM Compiler Infrastructure, release |release|. Here we
describe the status of Clang in some detail, including major
improvements from the previous release and new feature work. For the
general LLVM release notes, see `the LLVM
documentation <https://llvm.org/docs/ReleaseNotes.html>`_. For the libc++ release notes,
see `this page <https://libcxx.llvm.org/ReleaseNotes.html>`_. All LLVM releases
may be downloaded from the `LLVM releases web site <https://llvm.org/releases/>`_.

For more information about Clang or LLVM, including information about the
latest release, please see the `Clang Web Site <https://clang.llvm.org>`_ or the
`LLVM Web Site <https://llvm.org>`_.

Potentially Breaking Changes
============================
These changes are ones which we think may surprise users when upgrading to
Clang |release| because of the opportunity they pose for disruption to existing
code bases.


C/C++ Language Potentially Breaking Changes
-------------------------------------------

- The default extension name for PCH generation (``-c -xc-header`` and ``-c
  -xc++-header``) is now ``.pch`` instead of ``.gch``.

C++ Specific Potentially Breaking Changes
-----------------------------------------

ABI Changes in This Version
---------------------------
- Following the SystemV ABI for x86-64, ``__int128`` arguments will no longer
  be split between a register and a stack slot.

What's New in Clang |release|?
==============================
Some of the major new features and improvements to Clang are listed
here. Generic improvements to Clang as a whole or to its underlying
infrastructure are described first, followed by language-specific
sections with improvements to Clang's support for those languages.

C++ Language Changes
--------------------

C++20 Feature Support
^^^^^^^^^^^^^^^^^^^^^

C++23 Feature Support
^^^^^^^^^^^^^^^^^^^^^

C++2c Feature Support
^^^^^^^^^^^^^^^^^^^^^

- Implemented `P2169R4: A nice placeholder with no name <https://wg21.link/P2169R4>`_. This allows using ``_``
  as a variable name multiple times in the same scope and is supported in all C++ language modes as an extension.
  An extension warning is produced when multiple variables are introduced by ``_`` in the same scope.
  Unused warnings are no longer produced for variables named ``_``.
  Currently, inspecting placeholders variables in a debugger when more than one are declared in the same scope
  is not supported.

  .. code-block:: cpp

    struct S {
      int _, _; // Was invalid, now OK
    };
    void func() {
      int _, _; // Was invalid, now OK
    }
    void other() {
      int _; // Previously diagnosed under -Wunused, no longer diagnosed
    }

- Attributes now expect unevaluated strings in attributes parameters that are string literals.
  This is applied to both C++ standard attributes, and other attributes supported by Clang.
  This completes the implementation of `P2361R6 Unevaluated Strings <https://wg21.link/P2361R6>`_


Resolutions to C++ Defect Reports
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

C Language Changes
------------------
- ``structs``, ``unions``, and ``arrays`` that are const may now be used as
  constant expressions.  This change is more consistent with the behavior of
  GCC.

C23 Feature Support
^^^^^^^^^^^^^^^^^^^
- Clang now accepts ``-std=c23`` and ``-std=gnu23`` as language standard modes,
  and the ``__STDC_VERSION__`` macro now expands to ``202311L`` instead of its
  previous placeholder value. Clang continues to accept ``-std=c2x`` and
  ``-std=gnu2x`` as aliases for C23 and GNU C23, respectively.
- Clang now supports `requires c23` for module maps.

Non-comprehensive list of changes in this release
-------------------------------------------------

New Compiler Flags
------------------

Deprecated Compiler Flags
-------------------------

Modified Compiler Flags
-----------------------

* ``-Woverriding-t-option`` is renamed to ``-Woverriding-option``.
* ``-Winterrupt-service-routine`` is renamed to ``-Wexcessive-regsave`` as a generalization

Removed Compiler Flags
-------------------------

* ``-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang`` has been removed.
  It has not been needed to enable ``-ftrivial-auto-var-init=zero`` since Clang 16.

Attribute Changes in Clang
--------------------------
- On X86, a warning is now emitted if a function with ``__attribute__((no_caller_saved_registers))``
  calls a function without ``__attribute__((no_caller_saved_registers))``, and is not compiled with
  ``-mgeneral-regs-only``
- On X86, a function with ``__attribute__((interrupt))`` can now call a function without
  ``__attribute__((no_caller_saved_registers))`` provided that it is compiled with ``-mgeneral-regs-only``

- When a non-variadic function is decorated with the ``format`` attribute,
  Clang now checks that the format string would match the function's parameters'
  types after default argument promotion. As a result, it's no longer an
  automatic diagnostic to use parameters of types that the format style
  supports but that are never the result of default argument promotion, such as
  ``float``. (`#59824: <https://github.com/llvm/llvm-project/issues/59824>`_)

Improvements to Clang's diagnostics
-----------------------------------
- Clang constexpr evaluator now prints template arguments when displaying
  template-specialization function calls.
- Clang contexpr evaluator now displays notes as well as an error when a constructor
  of a base class is not called in the constructor of its derived class.
- Clang no longer emits ``-Wmissing-variable-declarations`` for variables declared
  with the ``register`` storage class.
- Clang's ``-Wtautological-negation-compare`` flag now diagnoses logical
  tautologies like ``x && !x`` and ``!x || x`` in expressions. This also
  makes ``-Winfinite-recursion`` diagnose more cases.
  (`#56035: <https://github.com/llvm/llvm-project/issues/56035>`_).
- Clang constexpr evaluator now diagnoses compound assignment operators against
  uninitialized variables as a read of uninitialized object.
  (`#51536 <https://github.com/llvm/llvm-project/issues/51536>`_)
- Clang's ``-Wfortify-source`` now diagnoses ``snprintf`` call that is known to
  result in string truncation.
  (`#64871: <https://github.com/llvm/llvm-project/issues/64871>`_).
  Also clang no longer emits false positive warnings about the output length of
  ``%g`` format specifier and about ``%o, %x, %X`` with ``#`` flag.
- Clang now emits ``-Wcast-qual`` for functional-style cast expressions.
- Clang no longer emits irrelevant notes about unsatisfied constraint expressions
  on the left-hand side of ``||`` when the right-hand side constraint is satisfied.
  (`#54678: <https://github.com/llvm/llvm-project/issues/54678>`_).

Bug Fixes in This Version
-------------------------
- Fixed an issue where a class template specialization whose declaration is
  instantiated in one module and whose definition is instantiated in another
  module may end up with members associated with the wrong declaration of the
  class, which can result in miscompiles in some cases.
- Fix crash on use of a variadic overloaded operator.
  (`#42535 <https://github.com/llvm/llvm-project/issues/42535>`_)
- Fix a hang on valid C code passing a function type as an argument to
  ``typeof`` to form a function declaration.
  (`#64713 <https://github.com/llvm/llvm-project/issues/64713>`_)
- Clang now reports missing-field-initializers warning for missing designated
  initializers in C++.
  (`#56628 <https://github.com/llvm/llvm-project/issues/56628>`_)
- Clang now respects ``-fwrapv`` and ``-ftrapv`` for ``__builtin_abs`` and
  ``abs`` builtins.
  (`#45129 <https://github.com/llvm/llvm-project/issues/45129>`_,
  `#45794 <https://github.com/llvm/llvm-project/issues/45794>`_)
- Fixed an issue where accesses to the local variables of a coroutine during
  ``await_suspend`` could be misoptimized, including accesses to the awaiter
  object itself.
  (`#56301 <https://github.com/llvm/llvm-project/issues/56301>`_)
  The current solution may bring performance regressions if the awaiters have
  non-static data members. See
  `#64945 <https://github.com/llvm/llvm-project/issues/64945>`_ for details.
- Clang now prints unnamed members in diagnostic messages instead of giving an
  empty ''. Fixes
  (`#63759 <https://github.com/llvm/llvm-project/issues/63759>`_)
- Fix crash in __builtin_strncmp and related builtins when the size value
  exceeded the maximum value representable by int64_t. Fixes
  (`#64876 <https://github.com/llvm/llvm-project/issues/64876>`_)
- Fixed an assertion if a function has cleanups and fatal erors.
  (`#48974 <https://github.com/llvm/llvm-project/issues/48974>`_)
- Clang now emits an error if it is not possible to deduce array size for a
  variable with incomplete array type.
  (`#37257 <https://github.com/llvm/llvm-project/issues/37257>`_)
- Clang's ``-Wunused-private-field`` no longer warns on fields whose type is
  declared with ``[[maybe_unused]]``.
  (`#61334 <https://github.com/llvm/llvm-project/issues/61334>`_)
- For function multi-versioning using the ``target``, ``target_clones``, or
  ``target_version`` attributes, remove comdat for internal linkage functions.
  (`#65114 <https://github.com/llvm/llvm-project/issues/65114>`_)
- Clang now reports ``-Wformat`` for bool value and char specifier confusion
  in scanf. Fixes
  (`#64987 <https://github.com/llvm/llvm-project/issues/64987>`_)
- Support MSVC predefined macro expressions in constant expressions and in
  local structs.
- Correctly parse non-ascii identifiers that appear immediately after a line splicing
  (`#65156 <https://github.com/llvm/llvm-project/issues/65156>`_)
- Clang no longer considers the loss of ``__unaligned`` qualifier from objects as
  an invalid conversion during method function overload resolution.
- Fix parser crash when dealing with ill-formed objective C++ header code. Fixes
  (`#64836 <https://github.com/llvm/llvm-project/issues/64836>`_)

Bug Fixes to Compiler Builtins
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Bug Fixes to Attribute Support
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Bug Fixes to C++ Support
^^^^^^^^^^^^^^^^^^^^^^^^

- Clang limits the size of arrays it will try to evaluate at compile time
  to avoid memory exhaustion.
  This limit can be modified by `-fconstexpr-steps`.
  (`#63562 <https://github.com/llvm/llvm-project/issues/63562>`_)

- Fix a crash caused by some named unicode escape sequences designating
  a Unicode character whose name contains a ``-``.
  (Fixes `#64161 <https://github.com/llvm/llvm-project/issues/64161>`_)

- Fix cases where we ignore ambiguous name lookup when looking up members.
  (`#22413 <https://github.com/llvm/llvm-project/issues/22413>`_),
  (`#29942 <https://github.com/llvm/llvm-project/issues/29942>`_),
  (`#35574 <https://github.com/llvm/llvm-project/issues/35574>`_) and
  (`#27224 <https://github.com/llvm/llvm-project/issues/27224>`_).

- Clang emits an error on substitution failure within lambda body inside a
  requires-expression. This fixes:
  (`#64138 <https://github.com/llvm/llvm-project/issues/64138>`_).

- Update ``FunctionDeclBitfields.NumFunctionDeclBits``. This fixes:
  (`#64171 <https://github.com/llvm/llvm-project/issues/64171>`_).

- Expressions producing ``nullptr`` are correctly evaluated
  by the constant interpreter when appearing as the operand
  of a binary comparison.
  (`#64923 <https://github.com/llvm/llvm-project/issues/64923>`_)

- Fix a crash when an immediate invocation is not a constant expression
  and appear in an implicit cast.
  (`#64949 <https://github.com/llvm/llvm-project/issues/64949>`_).

- Fix crash when parsing ill-formed lambda trailing return type. Fixes:
  (`#64962 <https://github.com/llvm/llvm-project/issues/64962>`_) and
  (`#28679 <https://github.com/llvm/llvm-project/issues/28679>`_).

- Fix a crash caused by substitution failure in expression requirements.
  (`#64172 <https://github.com/llvm/llvm-project/issues/64172>`_) and
  (`#64723 <https://github.com/llvm/llvm-project/issues/64723>`_).

- Fix crash when parsing the requires clause of some generic lambdas.
  (`#64689 <https://github.com/llvm/llvm-project/issues/64689>`_)

- Fix crash when the trailing return type of a generic and dependent
  lambda refers to an init-capture.
  (`#65067 <https://github.com/llvm/llvm-project/issues/65067>`_ and
  `#63675 <https://github.com/llvm/llvm-project/issues/63675>`_)

- Clang now properly handles out of line template specializations when there is
  a non-template inner-class between the function and the class template.
  (`#65810 <https://github.com/llvm/llvm-project/issues/65810>`_)

- Clang now properly converts static lambda call operator to function
  pointers on win32.
  (`#62594 <https://github.com/llvm/llvm-project/issues/62594>`_)

Bug Fixes to AST Handling
^^^^^^^^^^^^^^^^^^^^^^^^^
- Fixed an import failure of recursive friend class template.
  `Issue 64169 <https://github.com/llvm/llvm-project/issues/64169>`_
- Remove unnecessary RecordLayout computation when importing UnaryOperator. The
  computed RecordLayout is incorrect if fields are not completely imported and
  should not be cached.
  `Issue 64170 <https://github.com/llvm/llvm-project/issues/64170>`_

Miscellaneous Bug Fixes
^^^^^^^^^^^^^^^^^^^^^^^

Miscellaneous Clang Crashes Fixed
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
- Fixed a crash when parsing top-level ObjC blocks that aren't properly
  terminated. Clang should now also recover better when an @end is missing
  between blocks.
  `Issue 64065 <https://github.com/llvm/llvm-project/issues/64065>`_
- Fixed a crash when check array access on zero-length element.
  `Issue 64564 <https://github.com/llvm/llvm-project/issues/64564>`_

Target Specific Changes
-----------------------

AMDGPU Support
^^^^^^^^^^^^^^
- Use pass-by-reference (byref) in stead of pass-by-value (byval) for struct
  arguments in C ABI. Callee is responsible for allocating stack memory and
  copying the value of the struct if modified. Note that AMDGPU backend still
  supports byval for struct arguments.

X86 Support
^^^^^^^^^^^

- Added option ``-m[no-]evex512`` to disable ZMM and 64-bit mask instructions
  for AVX512 features.

Arm and AArch64 Support
^^^^^^^^^^^^^^^^^^^^^^^

Windows Support
^^^^^^^^^^^^^^^
- Fixed an assertion failure that occurred due to a failure to propagate
  ``MSInheritanceAttr`` attributes to class template instantiations created
  for explicit template instantiation declarations.

- The ``-fno-auto-import`` option was added for MinGW targets. The option both
  affects code generation (inhibiting generating indirection via ``.refptr``
  stubs for potentially auto imported symbols, generating smaller and more
  efficient code) and linking (making the linker error out on such cases).
  If the option only is used during code generation but not when linking,
  linking may succeed but the resulting executables may expose issues at
  runtime.

LoongArch Support
^^^^^^^^^^^^^^^^^

RISC-V Support
^^^^^^^^^^^^^^
- Unaligned memory accesses can be toggled by ``-m[no-]unaligned-access`` or the
  aliases ``-m[no-]strict-align``.

CUDA/HIP Language Changes
^^^^^^^^^^^^^^^^^^^^^^^^^

CUDA Support
^^^^^^^^^^^^

AIX Support
^^^^^^^^^^^

- Introduced the ``-maix-small-local-exec-tls`` option to produce a faster
  access sequence for local-exec TLS variables where the offset from the TLS
  base is encoded as an immediate operand.
  This access sequence is not used for TLS variables larger than 32KB, and is
  currently only supported on 64-bit mode.

WebAssembly Support
^^^^^^^^^^^^^^^^^^^

AVR Support
^^^^^^^^^^^

DWARF Support in Clang
----------------------

Floating Point Support in Clang
-------------------------------
- Add ``__builtin_elementwise_log`` builtin for floating point types only.
- Add ``__builtin_elementwise_log10`` builtin for floating point types only.
- Add ``__builtin_elementwise_log2`` builtin for floating point types only.
- Add ``__builtin_elementwise_exp`` builtin for floating point types only.
- Add ``__builtin_elementwise_exp2`` builtin for floating point types only.
- Add ``__builtin_set_flt_rounds`` builtin for X86, x86_64, Arm and AArch64 only.
- Add ``__builtin_elementwise_pow`` builtin for floating point types only.
- Add ``__builtin_elementwise_bitreverse`` builtin for integer types only.
- Add ``__builtin_elementwise_sqrt`` builtin for floating point types only.
- ``__builtin_isfpclass`` builtin now supports vector types.
- ``#pragma float_control(precise,on)`` enables precise floating-point
  semantics. If ``math-errno`` is disabled in the current TU, clang will
  re-enable ``math-errno`` in the presense of
  ``#pragma float_control(precise,on)``.
- Add ``__builtin_exp10``, ``__builtin_exp10f``,
  ``__builtin_exp10f16``, ``__builtin_exp10l`` and
  ``__builtin_exp10f128`` builtins.

AST Matchers
------------
- Add ``convertVectorExpr``.
- Add ``dependentSizedExtVectorType``.
- Add ``macroQualifiedType``.

clang-format
------------
- Add ``AllowBreakBeforeNoexceptSpecifier`` option.

libclang
--------

- Exposed arguments of ``clang::annotate``.

Static Analyzer
---------------

- Added a new checker ``core.BitwiseShift`` which reports situations where
  bitwise shift operators produce undefined behavior (because some operand is
  negative or too large).

.. _release-notes-sanitizers:

Sanitizers
----------

- ``-fsanitize=signed-integer-overflow`` now instruments ``__builtin_abs`` and
  ``abs`` builtins.

Python Binding Changes
----------------------

Additional Information
======================

A wide variety of additional information is available on the `Clang web
page <https://clang.llvm.org/>`_. The web page contains versions of the
API documentation which are up-to-date with the Git version of
the source code. You can access versions of these documents specific to
this release by going into the "``clang/docs/``" directory in the Clang
tree.

If you have any questions or comments about Clang, please feel free to
contact us on the `Discourse forums (Clang Frontend category)
<https://discourse.llvm.org/c/clang/6>`_.