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Clang |release| |ReleaseNotesTitle|

Written by the LLVM Team

.. 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. For the libc++ release notes, see this page. All LLVM releases may be downloaded from the LLVM releases web site.

For more information about Clang or LLVM, including information about the latest release, please see the Clang Web Site or the LLVM Web Site.

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

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. 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.

    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

Resolutions to C++ Defect Reports

  • Implemented CWG1473 which allows spaces after operator"". Clang used to err on the lack of space when the literal suffix identifier was invalid in all the language modes, which contradicted the deprecation of the whitespaces. Also turn on -Wdeprecated-literal-operator by default in all the language modes.

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.

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.

Removed Compiler Flags

Attribute Changes in Clang

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:).

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)

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)
  • Fix a crash caused by some named unicode escape sequences designating a Unicode character whose name contains a -. (Fixes #64161)
  • Fix cases where we ignore ambiguous name lookup when looking up members. (#22413), (#29942), (#35574) and (#27224).
  • Clang emits an error on substitution failure within lambda body inside a requires-expression. This fixes: (#64138).
  • Update FunctionDeclBitfields.NumFunctionDeclBits. This fixes: (#64171).

Bug Fixes to AST Handling

  • Fixed an import failure of recursive friend class template. Issue 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

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
  • Fixed a crash when check array access on zero-length element. Issue 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

Arm and AArch64 Support

Windows Support

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

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.

AST Matchers

  • Add convertVectorExpr.
  • Add dependentSizedExtVectorType.
  • Add macroQualifiedType.

clang-format

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).

Sanitizers

Python Binding Changes

Additional Information

A wide variety of additional information is available on the Clang web page. 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).